Mervyn Johnston sips his tea while sizing up the pristine-looking 1967 Mini Cooper that has come in for repairs from across the border. As the UK’s historic decision to quit the EU plays out, it doesn’t take much for the softly spoken 78-year-old and five-times rally-driving champion to cast his mind back to the days when customs posts and army checkpoints brought life in the picturesque village of Pettigo to a halt.

“We had about half a dozen incendiary bombs before the big one,” he says, tilting his chin to the other classic-cars garage across the road, now run by his son. “That blew the garage right into the river.”

Q&A Why is Dublin opposed to the idea of a hard border? Show Hide Ireland’s taoiseach, Leo Varadkar, has been much more sceptical than the UK about the potential for avoiding border posts via virtual checks on importers. Whilst agreeing with British ministers and EU negotiators that it is inconceivable for there to be a return to a hard border with the north, Dublin argues that the best way for the UK to achieve this would be by permanently remaining in a customs union with the EU and seeking single market membership like Norway through the European Economic Area. The UK has conceded that some of this will be necessary in its interim phase after Brexit, but hopes clever technological solutions can allow it have looser economic links in the long run. Varadkar is not alone in being sceptical about whether such a cake-and-eat-it customs and trade strategy is viable.

Pettigo is unique in Ireland as it is the only village divided by the border after Ireland gained independence from Britain in 1922. The river that runs beneath his workshop window places Johnston’s Protestant family in Northern Ireland and his largely Catholic neighbours on the other side of the 1820s cut-stone bridge in the republic of Ireland.

This village has seen more than its fair share of pain. Just to be associated with Britain during the Troubles of the late 60s to 90s was to be a target. “One day, a couple of IRA men came to the garage. I was held at gunpoint. They walked me up the road and held me at the customs post,” he says, pointing to the stone wall outside his garage.

“It wasn’t because I was a Protestant, it was because I was in the UDR [the part-time Ulster Defence Regiment, which supported the British army]. They fired at me. It was a bad shot and the bullet went under my arm and hit my apprentice. I skedaddled up the road and they fired after me. It was very frightening, but I got to my house and came back and got a weapon and fired a few shots back,” says Johnston.

Facebook Twitter Pinterest Mervyn Johnson in his garage in Pettigo. Photograph: Dan Dennison for the Guardian

That is just how it was back then. His attackers didn’t even wear balaclavas. “But they weren’t locals. I don’t think anyone was ever apprehended,” he says.

There were 28 “incidents” in this village, including several fatalities. One man was shot by the SAS; another died of his injuries in an incident the same day Johnston was attacked. A neighbour of Johnston’s was gunned down by the IRA as he tended his cattle.

Thanks to the arrival of the single market in 1993 and the Good Friday agreement in 1998, the border in Pettigo is now invisible, apart from a sign advising that speed limits switch from miles to kilometres at the old yellow-and-green customs shed on the Donegal side of the bridge. Cross-border trade has flourished since peace and, although it is not on a main border road, the village is blighted by the rumble of trucks carrying everything from freshly cut timber to Magners cider across the interstate bridge.

“We definitely don’t want a hard border here as we had one before, but if we do get one, there’s going to be hassle and I’d say trouble as well,” says Johnston.

Brexit has jolted memories and generated new anxieties along the 310-mile border from Donegal to Louth and what locals fear most is that old divisions and enmities will be stoked. It is estimated that one million people live in the border communities, and they are already feeling the impact.

“What you don’t see here is the silent hurt – everyone here was affected,” says Natasha McGrath, community development officer at the Termon Complex, a cross-border sports and community hall across the river. The EU spent €8.3m (£7.4m) on the centre, the second-biggest single beneficiary of peace programme funds after the Peace bridge in Derry.

McGrath fears that the EU funds released after the Good Friday agreement, which have underpinned peace, are now at risk. “We have spent the last two decades building bonds, socially, culturally, economically. The two states are bound together and now they are going to be cut apart,” she says.

Facebook Twitter Pinterest Pettigo’s border crossing: it runs across the short bridge in the centre of this photograph. Photograph: Dan Dennison for the Guardian

It is a fear that is felt throughout the shops and businesses of Pettigo. “You always have to worry when you divide a community,” says Martin Eves, who runs Enviro Grind, a successful compost and wood processing plant, just a few miles down the road on the Donegal side. “Building bridges takes years and, in my opinion, we are doing great. It’s not perfect, but to introduce division now is a complete retrograde step. It is not good to segregate people.”

His business will be affected, in ways he cannot predict. He is not sure his contracts with councils in Northern Ireland will be left intact after Brexit – EU rules may mean that every load of compost from over the border in Fermanagh must be declared.

“It’s the Berlin Wall approaching us here,” he says, adding that those sitting on green leather seats in Westminster “don’t care about Northern Ireland”.

With 18 backroads criss-crossing the Pettigo hinterland, he predicts smuggling will flourish post-Brexit, another worry for businesses. “I can guarantee you that will happen within a half-hour of the Brexit agreement being signed,” says Eves. “There are plenty of rogues and cowboys out there only too happy to get going.”

Petrol, cigarettes and heating fuel were the mainstays during the Troubles, with some industrial-scale border operations linked to the IRA in south Armargh’s “bandit country”. The head of those operations, Thomas “Slab” Murphy, allegedly amassed a fortune through fuel laundering and smuggling, estimated by BBC’s Underworld Rich List at between £35m and £40m in 2004.

Patrolling the border was notoriously difficult and suggestions of cameras on every road to monitor movements of goods and people after Brexit seems unworkable. In the old days, the back roads were spiked, blocked or blown up by the British army to stop smuggling or paramilitary activity.

“I remember sometimes there would be a massive crater in the road when we’d go see my grandad, because the army would have blown it up,” says Kevin Leonard, 34, a vet who works in Enniskillen and who certifies animals for cross-border slaughter on a daily basis. “It would mean a 10-mile diversion. Sometimes we’d just go across the river on the tractor.”

The Irish government has identified 11 “national” routes (not including Pettigo) involving the frontier, one crossing the border twice and the N54 crossing three times. The tax authorities in Dublin have suggested that eight of these could have customs stations. But there at least 200 more crossings to consider. Ray O’Leary, assistant secretary in Ireland’s department of transport, told a conference in Dublin this week: “There are twice as many crossing points [into the EU] as from the Arctic to the Black Sea. That’s an extraordinary thing to think about – Donegal to Louth has more crossings than the line from the north of Europe to the south of Europe.”

The N54 cuts across the border at three points. Illustration: Guardian Graphics

The issue with the border created by the partition of Ireland in 1922 is that it is so random, zig-zagging its way around the six counties in an unruly fashion. “The territorial divisions can be traced back to old clan loyalties. The system of 32 counties, established by the British administration in Dublin, was in position in the 17th century and has not changed since,” says James Anderson, professor of political geography at Queen’s University Belfast. He says the boundaries of counties are likely to have followed old Gaelic chieftains’ territories and maybe turf-cutting rights, explaining why places such as Rosslea and Clonooney are surrounded on three sides by a foreign state.

The fears now being expressed in Pettigo are set against a backdrop of tense politicking over the border, which has escalated in the last fortnight. Ireland and the rest of the EU maintain that the only way to avoid a hard border is for Northern Ireland to stay in the customs union and single market, even if the UK pulls out.

Arlene Foster, leader of the Democratic Unionist Party (DUP), which is propping up the Conservative government, is adamant Northern Ireland must quit the customs union and this week accused Brussels and Dublin of trying to “blackmail” Brexit negotiators.

For the pro-Brexit media, it is good knockabout material for pungent anti-EU editorials, with the Sun labelling the taoiseach a “Brexit buffoon” who should “keep his gob shut”. For people on the border, the war of words simply underlines how voiceless their communities feel.

“We’re forgotten as it is, here in Pettigo,” says McGrath. “I think [a hard border] will socially exclude us. Theresa May hasn’t a notion. I don’t know if they give a crap about us on the border, and that’s my polite way of putting it.”

Some 60 miles away, in Armagh, the Centre of Cross Border Studies is holding a conference raising just these worries. Of major concern to its speakers is the continued stability of the Good Friday agreement, which spawned a web of cross-border initiatives on everything from health to fisheries, sport to security. The EU spent the summer mapping out those activities and has now documented 142 areas touched by EU law, now in jeopardy, including cross-border health schemes.

Facebook Twitter Pinterest Natasha McGrath: ‘We’re forgotten as it is, here in Pettigo.’ Photograph: Lisa O'Carroll for the Guardian

“People who have worked in cross-border co-operation say it is a game-changer,” says Pamela Arthurs, chief executive of the East Border Region, a cross-border community initiative. “The potential for Brexit to go back to the bad old days, when ambulances were stopped at the border and patients taken out of one ambulance and put in another, is there,” she tells delegates.

It is through the work of people such as Arthurs, who have devoted their lives to improving life on the border, that cancer patients in Donegal (in the Republic) can now get treatment in Derry (in Northern Ireland), while children in “the north” can access cardio services in Dublin. “Brexit is not just the topic for the dinner table here. It is real life, and here, in the east of Northern Ireland, it looks like it’s going to affect us in a bad way,” Arthurs says.

The Irish Central Border Area Network surveyed bordered communities for a report submitted to the EU last week. “What people are saying is that any route to [a hard] border is a route to the past that people are afraid of,” says its chief executive, Shane Campbell. “Anything that goes back is seen as a failure.”

One of the reasons border people in the north, who voted overwhelmingly to remain in the EU, feel unheard is the fact they are not represented in either regional or national government. For the past year there has been no government in Northern Ireland, and Sinn Féin, which won every border constituency in the 2017 general election, does not take up its seats in Westminster.

Facebook Twitter Pinterest A mother reflects on Brexit’s consequences for Northern Ireland in Your Ma’s a Hard Brexit, a film made by the Guardian and Headlong

“In my view, people in Antrim [an area loyal to the union of Britain and Northern Ireland] couldn’t care less about the border, but they will find themselves affected,” says Anderson. “Unionists are fond of saying most of our trade is not with the south but with Britain to the east, but it is quite possible that milk from cows in Ballymoney or Coleraine goes south.”

Agrifood is an all-island single market – a third of the milk from cows in Northern Ireland goes south for production into butter, cheese and infant formula. Many of the big household names have cross-border operations, including Guinness and Baileys Irish cream. According to the book Brexit and Ireland, by RTE broadcaster Tony Connelly, Baileys’ parent company, Diageo, buys 275m litres of milk from 40,000 dairy cows on 1,500 accredited farms north and south of the border.

It is a result of this barrier-free trade that large-animal vet Kevin Leonard has carved a livelihood doing the EU paperwork on farms in county Fermanagh. “This morning, I looked at 298 lambs that were going to be slaughtered in county Kildare tomorrow,” he says. “I checked they were healthy, checked the paperwork, that they were all tagged and they were fit to travel.”

He is concerned that Brexit will destroy the farms in the area. “People are eating less lamb; they want food that cooks quickly, ready meals. Almost half the lambs reared in the north of Ireland, around 300,000 a year, go to an abattoir just outside Dublin because there is no capacity in Northern Ireland. About 70% of those go to France,” he says.

Facebook Twitter Pinterest An old border customs post in the Republic. Photograph: Dan Dennison for the Guardian

As he fans out 11 pages documenting the animals’ origin, age, breed and other details, such as confirmation the truck is washed out with an officially approved disinfectant beforehand, he explains that he is a small farmer himself. He doesn’t understand why any farmers voted for Brexit – 87% of their income comes from EU subsidies and they will be wiped out if that money is not replaced.

Then there is the issue of farm inspections – some straddle the border, with stories of farms with “their back door in the north and the front door in the south”. One of the partners in his practice is from the border town of Belcoo, 20 minutes away, and needs to cross the frontier “three or four times a day just to get to work”.

Belcoo, which runs into another village, Blacklion, on the south of the border, was on one of the old “approved roads” for freight, as was Rosslea further east, where army watchtowers practically cut off the town before the single market in 1993.

Gerry Scott, 59, a builders’ merchant on Rosslea’s high street, sketches out the border on a pad in his back office. His shop is in “the north”, but the woodland visible from his office window is “the south”, as is the land behind him and to the right. “We are surrounded, like an island. I have customers from all over. I hope communities here won’t be forgotten. We employ eight people and a couple of people part-time. Now, we can trade for 360 degrees; if there’s a hard border we can only do 180 degrees,” he says.

Half an hour away in the Republic, peace has brought bustling business in the agrifood sector, with Monaghan to the south now the mushroom capital, from which millions of punnets are transported daily to supermarkets across Britain.

A few miles north of the border in Ballygawley, Elaine Shaw is surveilling a £25m plant under construction that will produce compost made up of straw and poultry droppings. She supplies specialist mushroom-growing medium for 25 growers on both sides of the border, and fears that Brexit will add prohibitive costs to the business. “Our worry is that the government could let this sector just dwindle away, and once that has gone, you’ll never get it back,” she says. “We are stronger together. We just worry that the DUP want Brexit and won’t yield anything on the customs union and single market.”

Facebook Twitter Pinterest David Parkes: has spent his life as an ‘in-betweener’. Photograph: Dan Dennison for the Guardian

The resilience of the border communities is a point of pride here. “Local people just learn to adapt; they are different because their life is all about adapting,” says David Parkes, a 25-year-old jockey, who is back from the UK to recuperate from an injury, in Middletown, a border crossroads on the northern side between Monaghan and Armagh.

One of the 11 national roads crossing the border passes through here, and the derelict remains of the customs checkpoint stands alongside Parkes’s family farm. Nowadays, Middletown is little more than a few houses, a garage and a money-changing cabin, whose businesses wax and wane according to the currency exchange rates. He notes that Monaghan, 10 miles away, is booming with dairy, mushroom and chicken farms, while noting that “it is sad” that Armagh, half an hour away, “is a bit hit and miss”.

“People on the border learn to sell and buy anything,” he says. “We could sell sand to the sheikhs. A lot of people are money-driven around here.” A Protestant, he explains that he has spent his life as an “in-betweener”, derided in a segregated school because he had Catholic friends.

He remembers the British army helicopters landing in his neighbours’ fields, the stories of shootings at the customs house, and the smuggling. He supports the suggestion that the new border could run down the Irish Sea, placing border and passport controls at airports and docks where there is room for vehicle inspections, and time for passport checks. “Or they could just do it on the other side, in England. That way you’d take away the power from the DUP. What could they complain about then?” he says.

Peace has been welcome, and now neither Parkes nor the rest of those living by the border know what is around the corner. “Border people, we are in a minority,” he says. “Someone is going to decide our lives at a stroke of a pen. We don’t have a voice in this.”