If you take a plane from the UK mainland to Belfast, you could believe that everything is roughly as it been since the Good Friday Agreement 20 years ago. It may be difficult, occasionally tense, but not impossible and, crucially, not at war.

There are “peace walls” topped with coiled barbed wire, kerbstones painted in rival colours and rival flags flying at what are called the “interfaces”. But this year, as the police noted, was the first in four decades that there were no bonfires in Belfast to mark the 8 August anniversary of internment. While hotter weather-wise, this summer has so far been cooler in other respects than many.

Say goodbye to Belfast, however, and take the bus west-northwest, to the city now called Derry-Londonderry (by those who prefer not to take sides), and you will soon be enveloped in a rather different vibe.

The bus – linking Northern Ireland’s two major cities – takes two hours, with an extra 15 minutes at present for roadworks. The train takes a little longer. Either way, it will take you longer to traverse the distance between Northern Ireland’s two biggest cities than it takes to fly from anywhere in the mainland to Belfast.

Traffic passes anti-Brexit signs on the County Derry/Londonderry Northern Ireland and County Donegal in the Irish Republic. Northern Ireland could be given joint EU and UK status and a 'buffer zone' on its border with the Republic, under new plans being drawn up by David Davis, according to reports. (EPA)

And when you look at the road and rail map of Northern Ireland, you can partly see why. There are reminders here of just-united Germany or, say, Washington DC – places whose fractured transport infrastructure betrays dislocation of a more profound kind. The lines may once have been intended to join up, but they don’t – and not just because the money ran out.

I went to Northern Ireland in the hope of catching something of the mood at what seems a rather crucial time in a place that is – potentially – the Brexit frontline. As it turned out, I was following, unwittingly, hard on the heels of some very important guests.

The previous 10 days had seen Theresa May give a speech in Belfast to defend her already moribund Chequers “deal”. It was a speech that pressed many loyalist buttons, while also recognising that many claimed Irish identity. But it was noted in the province for something else: in her speech, May had used the formulation “Derry-Londonderry” – apparently becoming the first British prime minister to do so from a public stage. This went unremarked on the mainland.

Northern Ireland police chief says he is taking responsibility for coordinating all of the difficulties with the border

A few days later, Philip Hammond, the chancellor, had made a separate trip to Derry-Londonderry to hold out the prospect of a so-called City Deal – an arrangement already widespread elsewhere in the UK, that gives local authorities more say in how central government money is spent. Belfast is still finalising its own City Deal, but Northern Ireland’s second city is now in line for one, too.

And third – belatedly, in the view of some – had come Karen Bradley, the Northern Ireland secretary, who toured the areas of both cities where street violence had erupted, seemingly out of the blue, in early July. Police blame the rioting on “new IRA” “dissidents” and it seemed to die down as suddenly as it had flared – suggesting that someone somewhere had the power to switch it on and off – but 70-plus petrol bombs in one night in the Derry Bogside, with children as young as eight involved, cannot be dismissed as nothing.

Was such a procession of senior British ministers to Northern Ireland unusual, I asked. Yes, it was – and it stirred memories for me of how senior UK politicians had rushed to Scotland 10 days before the 2014 referendum after a poll suddenly showed a turn towards independence. “Panic? What panic” was the official mantra then, and it could apply again.

So is the Westminster government finally waking up to the dangers that Brexit could present to Northern Ireland – and not just to Northern Ireland, but to the uneasy peace that prevails there, and even to the current composition of the United Kingdom? Arch-Brexiteers argue that there is no risk – indeed, that the whole issue is being inflated out of all proportion by Remainers in their flailing attempts to frustrate Brexit. Boris Johnson is reported to have applied the same profane dismissal to the concerns of Northern Ireland as he applied to the expressions of alarm from UK business.

I would respectfully recommend that they follow me past Belfast (which hogs the bulk of Westminster subventions to the province and stands to survive any adverse Brexit effect better than the rest) to Derry – I’m dropping the Londonderry for practical not political considerations – and just listen to what people say. It would not necessarily be what they expect.

Once upon a time you might have been struck by the sectarian divide, here as in Belfast, which would have split opinion on everything. This August, that was not my first, or overwhelming, impression. The alignments of Protestant/unionist/loyalist on the one side and Catholic/republican/nationalist on the other seem less clear than they were.

Whether you talk to engaged observers, such as Michael Gove’s much maligned “experts” or local journalists, to businesspeople, or to concerned individuals of the sort who crammed into talks organised for this – the 26th – year’s Feile (community festival), even if you just eavesdrop on people chatting among themselves in the walled city’s cafes and bars, you will not have to wait long before you hear such terms as “disaster”, “destruction”, “devastation”.

This is what they fear from a hard Brexit, from a no-deal Brexit, or from pretty much any Brexit that entails the reinstatement, however partial or “technical”, of the border that wends its way just a few miles/kilometres from their city. And they understand, with a stark clarity that seems totally lacking on the mainland, that when (not if) the UK leaves the European Union, there will be a land border with the Republic of Ireland, which will become a fully foreign country.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

For younger people, this will be for the first time in living memory.

You can talk about special “bilateral” arrangements as much as you like, about sophisticated Customs technology, or – wistfully – about drawing a new Customs border down the Irish Sea. But no one, it seemed to me, has any illusions. The Good Friday “fudge” that allowed the border to be visible for those who wanted to see it and invisible to those who did not, that allowed Northern Ireland’s population to choose to be British or Irish or both, is coming to an end.

Now you might object that this alarm – and it is alarm – stems largely from the fact that the UK government has not been doing a sufficiently good job of reassurance. Maybe. But if the UK government in Westminster isn’t profoundly worried by now about the Brexit effect on Northern Ireland, then it should be.

The economic writing is already on the wall. The fall in the pound against the euro is not benefiting Northern Ireland. It is sending skilled EU workers home, or across the border. Uncertainty is killing investment. And it is not as though Northern Ireland was flourishing to start with. The Good Friday dividend never really arrived north of the border, and the comparative deprivation beyond Belfast is a shocking indictment of decades of poor government from London and Stormont. The northwest feels as abandoned by the UK as it ever has.

Which is partly why the political writing is on the wall, too. The rioting that broke out early last month – in places where rioting tends to break out – was passed over as an unfortunate little local difficulty. Well, maybe. But levels of violence can also be a gauge of something else: morale, discontent, frustration. What was the trigger? Will it recur? And one reason why local politicians took such a dim view of the Northern Ireland secretary’s tardiness in visiting was that the leader of the DUP, Arlene Foster, and the members of the currently defunct Stormont Assembly and the Irish foreign minister, Simon Coveney, had made a joint visit to the area more than a week before. What does that say about the engagement of the “mother country”?

But something else is happening, too, which casts a slightly different light on this otherwise depressing scene. Late last month, after the week of riots, after the procession of UK visitors, the former DUP leader, now elder statesman, Peter Robinson used a conference speech just across the Irish border to reopen the taboo question of a united Ireland. In remarks received furiously by some fellow unionists, Robinson said he did not think Northern Ireland would vote to leave the UK, but people should “prepare” for the possibility and “accept the result”.

It turned out that he was giving public voice to a di​scussion that had already begun, and that discussion exposes very different concerns from the past. It used to be that the power of the Catholic Church was the overriding objection. With the reputation of the church tarnished by child abuse scandals and social attitudes, both among the public and enshrined in law, more liberal than in much of the north, the arguments now are about the place of the British legacy, the economy, and… the NHS (that is, keeping a health service that is “free at the point of delivery”).

It is not clear where this might go. But the conversation now is not just about whether, but how. And allegiances appear to be more fluid, perhaps a lot more fluid, than they were – thanks to generational change in the north, social and economic change in the Republic, and, of course, Brexit.

The size of the majority for Remain in the EU referendum in Northern Ireland showed that it crossed the Catholic/Protestant, republican/unionist divide. A hard border, even the prospect of a hard border, could push the line even further. The result, in the event of a referendum on unification, may not be as predictable as it once was.