AUGHNACLOY, Northern Ireland — With nothing to mark the point where one country ends and another begins, there is little sign of the border between the United Kingdom and Ireland, a frontier that was once controlled by military checkpoints.
At the Diamond Bakery and Tearoom in Aughnacloy, customers can pay in British pounds or the euros used in Ireland, and visitors often ask the staff members which country they are in.
Glenn Robinson, who owns a nearby computer business, said that he had made two journeys into Ireland that day, and that one of his employees commutes to work from there. “You don’t see it as a border,” he added.
Yet this least intrusive of frontiers could become a lot more visible if Britain votes next week to leave the European Union, quitting a 28-nation bloc in which its neighbor, Ireland, will remain.
Customs posts could spring up again, as could some form of immigration controls to prevent migrants from using Northern Ireland as a backdoor route to the rest of Britain. And the possibility of a “hard” border with Ireland is just one of the issues thrown up for Britain’s regions by the vote on Thursday.
Polls suggest that voters in Scotland and Northern Ireland are likely to favor remaining in the European Union, while their English counterparts appear to be leaning in favor of leaving. (Views in Wales are thought to be consistent with those in England.)
Yet the Scots and the Northern Irish could find themselves unwillingly outside the European Union if their votes are outweighed by those of the more numerous English.
A British exit, known as Brexit, would most likely stoke renewed pressure in Scotland for independence from Britain — an idea rejected in a 2014 referendum but one that has not gone away.
A vote to leave the bloc would also put Northern Ireland on the external frontier of the European Union. And it would present challenges to the formal and informal integration of North and South that has accelerated in the wake of the complex 1998 peace agreement that defused the long sectarian conflict between pro-British Protestants and pro-Irish Catholics in the North.
In doing so, it could risk reversing a process during which, in the words of Mary McAleese, a former Irish president, customs and border controls have “melted away.” While Northern Ireland is unlikely to opt for new constitutional arrangements with the British government in London and the Irish government in Dublin anytime soon, were it to feel “the cold winds of a serious economic downturn” after a British divorce from Europe, there might be a “new political dynamic” there, she added.
Even a vote to remain could have constitutional implications if Scottish and Northern Irish votes swung a close result in favor of staying, antagonizing English supporters of leaving.
Such outcomes could force voters to re-examine what binds England, Scotland, Wales and Northern Ireland.
“The U.K. is in some ways inherently unstable because of the mixture of different national groupings, and also the asymmetry of the size of England relative to the others,” said Andrew Blick, lecturer in politics and contemporary history at King’s College London.
In Northern Ireland, which remained part of the United Kingdom after Ireland became independent nearly a century ago, the prospect of pulling out of the European Union has provoked fierce debate.
Two former British prime ministers, John Major, a Conservative, and Tony Blair, of the Labour Party, have predicted that withdrawal from Europe would mean the return of border controls between the North and the South, and could undermine a peace process that healed decades of violence in Northern Ireland. At risk, Mr. Major said, is “the complicated and multilayered constitutional settlement that underpins the present stability in Northern Ireland.”
His assertion was rejected as “highly irresponsible,” by Theresa Villiers, the secretary of state for Northern Ireland, a British cabinet member who supports leaving the European Union — unlike the leader of her Conservative Party, Prime Minister David Cameron, who wants Britain to remain. Ms. Villiers said she believed it would be possible to “keep an open land border” with Ireland.
The debate has stirred memories of life in Aughnacloy during the years known as the Troubles, when the sectarian conflict led to violence and a large British military presence.
Stephen Salley, an architect, grew up here in the days when it hosted a British Army base, when several fatal shootings took place, and when military helicopters often hovered overhead, sometimes causing domestic chaos by blowing soot down the chimney of his family home.
In those days Mr. Salley rarely traveled to Ireland, and he worries that today’s easy mobility could be impaired. “I don’t think it would be exactly the same as it is now — there would have to be more controls,” he said.
In Scotland, advocates of independence suggest that a decision to leave the European Union would generate political pressure for another referendum on Scotland breaking away from Britain. Scotland is now dominated politically by the Scottish National Party, which is dedicated to independence, though the party’s leader, Nicola Sturgeon, has been vague about whether she would move quickly to seek another referendum in the event of a decision by Britain to leave the European Union.
“If we are taken out of the E.U. then a lot of Scots would want to be taken out of the U.K.,” said Henry McLeish, a former first minister of Scotland from the opposition Labour Party.
Mr. McLeish said he also worried about a “clash of nationalisms,” stoked partly by the pro-Brexit, populist, U.K. Independence Party in England.
Yet, with most senior Scottish politicians wanting to remain in the European Union, the referendum appears a strangely muted affair in Scotland — a pale shadow of the independence plebiscite of 2014 that divided friends and families.
Humza Yousaf, a minister in the Scottish government and a rising star of the Scottish National Party, said Scots were turned off by the debate in England, with its focus on immigration, its warring among English Conservative politicians, and what he described as a “negative, smearing, name-calling, scaremongering type of campaign.”
Migration is not as hot an issue in Scotland as in England because fewer people have entered the country from the Continent, immigrants have been better dispersed and their labor is needed to sustain the Scottish population, said Mr. Yousaf, who was born in Scotland to immigrant parents.
He is more cautious than some colleagues, such as the former Scottish National Party leader Alex Salmond, who has predicted a rapid second referendum on Scottish independence if England “drags” Scotland out of the European Union.
In the 2014 Scottish independence referendum, it was assumed that both countries would stay in the European Union. Were England to quit the bloc while Scotland wished to remain, their relationship would be more complex.
“There are clearly questions that would be asked about the currency question, about the border question,” Mr. Yousaf said, adding that, while circumstances would be “different” from 2014, the potential complications could be successfully addressed.