A flag for the Republic of Ireland has to mark the mostly invisible border between Northern Ireland and the Republic of Ireland. (William Booth/The Washington Post)

Back during the Troubles, the border here was a treacherous place, a briar patch of watchtowers and customs posts, and rough smuggling clans and lethal Irish Republican Army cells.

A generation ago? So fearful were British troops of IRA snipers that they deployed their soldiers in helicopters instead of risking the roads in County Armagh.

Today, this is a fine place to be a cow.

Now there is peace — and plenty of golf being played — along the 300 miles of the sinuous border that separates Northern Ireland from Ireland.

Yet change may be coming to the frontier, following the vote in June by Britain to leave the European Union.

There are no signs, no custom posts or immigration control along the 300-mile invisible border between the Republic of Ireland and Northern Ireland. (William Booth/The Washington Post)

What will happen to the Irish isle, north and south, is one of the biggest wild cards of the Brexit vote.

Northern Ireland is a part of Britain, and so it must now bid goodbye to the European bloc, no matter that a clear majority in Northern Ireland wanted to stay in the union — 56 percent voted to remain, while 44 percent voted leave.

Their neighbors to the south in Ireland will remain part of the E.U.

What will happen to trade and travel is unknown — and there are even bigger questions being asked about unification of the island.

Will a Romanian — or a Libyan — traveling from Dublin soon have to show a passport on the way to Belfast? Will a truckload of E.U. or British goods be inspected crossing the border, and how? Will a bottle of milk cost the same on both sides — and who will enforce the regulations for its proper pasteurization, and what will happen to the millions in farm subsidies, tax breaks and development funds that help farmers produce the milk?

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Britain’s new post-Brexit prime minister, Theresa May, vowed that “no one wants a return to the borders of the past.” But many question what’s ahead.

Politicians in the U.K. insist that they will move forward with the process to exit the E.U., but not everyone is happy about it. Here's what will make it a long and difficult ordeal. (Jason Aldag/The Washington Post)

“Nobody knows what’s going to happen to our border, and people who know the least are the politicians,” said Eugene McSkeane, 39, a hog farmer in Crossmaglen in Northern Ireland, who pointed to the overblown promises made before the historic Brexit ballot that were quickly withdrawn after the votes were counted.

The farmer said the decision about the border won’t be made by just Britain or Ireland. The remaining members of the E.U. will also have a say.

McSkeane lives in the north but crosses back and forth across the border without a thought.

“We pay our electric bill in the south and our water bills in the north,” he said. “It’s second nature.”

Kids go to schools on either side. Farmers till land that straddles the line. A local veterinarian said it’s a morning’s work to treat a cow in the north and a sheep in the south.

“Technically, I imagine you’re supposed to check in with someone when you transport a body across the border, but I don’t see why you would bother with that now,” said Bernard O’Hanlon, 56, a funeral director and owner of a pub in Mullaghbawn in Northern Ireland. His two businesses, alongside a car wash, are combined.

“We’ve forgotten all about borders,” he said. “Now are they going to mean something again? That’s daft.”

At the crossroads near O’Hanlon’s is a monument to fallen fighters who died during the 1916 Easter Rising against British rule that led to independence for Ireland. Beneath the flowers and portraits are the words, “If you really want an Irish Republic vote Sinn Fein.”

‘A can of worms’

“Drive down the hill over the river you won’t see a sign telling you you just crossed a border,” said Brendan McAleavy, 55, a publican in Cullaville, whose bar has two different drawers at the cash register, one for British pounds, the other for euros. (Britain maintained the pound as its currency despite being an E.U. member, while Ireland adopted the euro).

More than 180 formal roads cross the border — many more if you count tractor trails and foot paths.

Along the River Fane, anglers fish for trout from both sides of the border. A local has hung the green, white and orange flag of Ireland along a hedgerow.

This will now become the E.U.’s back door to Britain and vice versa.

Alasdair McDonnell, a member of Parliament from Belfast, said he’s been deluged with queries from constituents worried about what will happen to the border in the wake of the Brexit vote.

“We’ve opened up a can of worms,” he warned during a debate in Parliament.

“There’s been massive progress and benefits of the last 20 years,” he said. “Free movement has transformed the island of Ireland.”

“There are people with a living memory of the hard border, and it’s not a good memory at all,” said McDonnell, a member of the Social Democratic and Labour Party.

“Nobody wants a return to the dark days,” he said.

It was only 15 years ago that the last bomb exploded in the long conflict between British security forces and Ulster loyalist paramilitary groups and the Irish Republican Army. More than 3,500 people were killed during the Troubles, half of them civilians.

The peace brought by the 1998 Good Friday Agreement is now well-worn, widely accepted and an economic success for both north and south.

For some, change is worrisome.

“Brexit got everyone talking, that’s for sure. It reminds everyone who is who, where is where, north and south, the Troubles, all of that,” said Anne Devlin, a shop clerk who was filling her car with gas in Castleblayney in Ireland but who lives in Northern Ireland.

During the Troubles, one side’s freedom fighter was the other’s terrorist, she said.

“The past is best in the past,” Devlin said.

“It doesn’t take much to stir tensions on the border,” said ­Eunan O’Halpin, a professor of contemporary Irish history at Trinity College in Dublin.

“Is there still bitterness? Of course there is,” he said.

[Theresa May vows ‘bold, new’ future for Britain as E.U. exit awaits]

A vote for a united Ireland?

After the Brexit ballot, Irish politicians quickly began to jostle for advantage as they assessed what future negotiations over the coming split would mean — for the border, for the relations between north and south, between London, Belfast, Dublin and Brussels.

“That Ireland can be both one unit and two separate units may be a bizarre political fiction, but it is a fiction that has enabled former enemies to live with one another in relative peace,” Ian McBride, professor of Irish and British history at King’s College in London, wrote in the Guardian.

In an interview, McBride said, “I assume there’ll be a common sense solution to this.” He said too many people have too much to lose.

The leader of the main opposition party in Ireland, Micheal Martin, said the rejection of Brexit by voters in Northern Ireland could be a “defining moment” in Irish politics and “may show people the need to rethink current arrangements.”

Martin McGuinness, deputy first minister of Northern Ireland and a leader of Sinn Fein, called Brexit “a disaster for Ireland.”

“Anything that resembled a return to border checkpoints would represent a grievous undermining of the Good Friday Agreement,” McGuinness said. “I view such a prospect with great alarm.”

Sinn Fein’s leader, Gerry Adams, said that the vote by Northern Ireland against Brexit should boost support for a future referendum on Irish unity.

Talk of such a “border poll” to consider the reunification of north and south raises hackles among British loyalists in the north.

Arlene Foster, first minister of Northern Ireland, vowed that little would change along the border and said talk of a referendum on the unification of Ireland was folly.

“This is the silly season, and often we have people coming forward with policy ideas that have no relevance to reality, and certainly a border poll, if it were to happen, would give a resounding result that we wanted to remain within the United Kingdom,” Foster told the BBC’s “Today” program.

Sammy Wilson, a leader of the Democratic Unionist Party, which supported a leave vote in the Brexit campaign, told The Washington Post in an interview at his offices in Larne that there are already immigration controls shared by Ireland and Britain in the form of the Common Travel Area protocol.

Wilson said the threat of “rising tensions” between Catholics and Protestants, republicans and unionists, is “a despicable argument.”

“We don’t have a disaffected population. We won’t have a return to terrorism,” he said.

There’s too much trade, too much togetherness now.

“It’s all going to blow over soon enough,” he said.

Read more:

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Today’s coverage from Post correspondents around the world