A defaced "Welcome to Northern Ireland" sign on the border in Middletown, Northern IrelandClodagh Kilcoyne / Reuters

DUBLIN—Few people were talking about Irish reunification in the spring of 2016: Most assumed that the Good Friday Agreement had put the issue on ice for the foreseeable future. Northern Ireland’s mainly Protestant pro-British majority and the Catholic minority, which traditionally favored reuniting with the Republic of Ireland, had been living in peace after decades of sectarian violence. Where was the urgent need for such a drastic change?

Then the unforeseen happened. The United Kingdom voted narrowly to leave the European Union. Suddenly, Northern Ireland—which, as part of the U.K., had voted to remain in the EU—was to be taken out of the bloc; the border between Northern Ireland and the Republic of Ireland, an EU member state, would once again be a tangible barrier to movement and trade after a protracted period of near-invisibility. Warnings proliferated of the consequences—economic, security, political—of separating the two sides of the island of Ireland. Chief among these was the risk that the border would again be a flashpoint for conflict, a resumption of the Troubles, which killed more than 3,500 people over 30 years.

In this newly febrile atmosphere, the idea that Ireland might someday be reunited, peeling Northern Ireland away from the U.K. and combining it with the republic, no longer seems so far-fetched.

Such a future is envisaged in the Good Friday Agreement, via referenda on both sides of the border, and public-opinion surveys and population trends suggest that this is more than just a remote possibility. For one thing, long-term demographic changes make it likely that Roman Catholics in Northern Ireland, most of whom identify as Irish nationalists, will soon become the largest religious group in the statelet, which was crafted in 1921 specifically to have a pro-British Protestant majority. And sentiment appears to be moving in that direction: Last month, for the first time, a survey found that a majority in Northern Ireland was in favor of reunifying, albeit only by a margin of 51–49. (The poll was, notably, carried out by a company owned by Michael Ashcroft, a former senior official in Britain’s Conservative Party, which is officially unionist.)

The notion has become so mainstream, in fact, that Peter Robinson—a former leader of the hard-line Democratic Unionist Party (DUP) whose dedication to preserving Northern Ireland’s place in the U.K. once led him to co-found an armed paramilitary group—publicly stated last year that unionists like him would be wise to start planning for a vote, and to be prepared to respect the result. “I don’t expect my own house to burn down,” he said, “but I still insure it because it could happen.”

It is hard to overstate what a remarkable shift this is. And yet, at the same time, it is difficult to understate how ill-prepared everyone is for it to actually happen.

How, practically, would Ireland be reunited? What flag would fly over which buildings? Would residents of Northern Ireland still be able to claim both British and Irish nationality? Would a united Ireland be a unitary state under Dublin’s present structure, reversing (or, some nationalists might say, avenging) Britain’s partition of the island almost a century ago, or would it be a newly federal country, with the north retaining its separate parliament, police, and administration and London standing as guarantor of the interests of Northern Ireland’s unionist population, as Dublin has stood for Northern Irish nationalists?

Such questions are already being debated quietly in many unofficial forums and back rooms, yet neither Dublin nor London is involved. The British government is consumed by Brexit, while Ireland says its priority is to protect the Good Friday Agreement (“The Irish government very firmly does not see Brexit as a vehicle for achieving a united Ireland—they are separate and distinct issues,” a government spokesman said). Even the Northern Irish assembly and executive, which have an array of their own powers, have been suspended for the past three years because of a standoff between the DUP and Sinn Fein, the main nationalist party. As far as is known, no unionist politician has yet joined any of these private conversations about Irish unity. And even those who are prepared to at least consider the possibility remain deeply skeptical, not only of arguments about identity and sovereignty, but of the practical benefits as well.

Chief among these is the economic case for Irish unity. In 2016, the Republic of Ireland, flush with tourism, high-tech industry, and foreign investment, had a per capita gross domestic product of €53,300 ($58,800). In Northern Ireland—the U.K.’s poorest region, largely dependent on British government spending, which London puts in excess of £9 billion ($11.5 billion) a year—the figure was €23,600.

But as Steve Aiken, an Ulster Unionist Party member of Northern Ireland’s assembly, notes, much of the region’s trade is with the rest of the U.K., so reunifying with “the south” would have a cost as well. And while incomes are higher in the republic, Aiken, a unionist so moderate that he has lived and worked in Dublin, argues that because of much higher prices and poorer public services, the standard of living in the republic is worse. Take the issue of health care: At present, residents of Northern Ireland can avail themselves of the U.K.’s National Health Service, which is free at the point of care, whereas the republic has a dysfunctional and expensive mixed public-private system.

Reverse arguments also apply. Peter Shirlow, the director of the University of Liverpool’s Institute of Irish Studies, says many in Dublin’s political and business elite have privately told him that Ireland cannot make up the multibillion-pound annual subsidy from London that currently sustains Northern Ireland’s economy.

“I think the people in the south would vote for unity right now because it’s emotional,” Shirlow told me, “but they haven’t had the debate yet about what it would cost.”

Then there is the issue of security. Extremist unionist paramilitary groups—which murdered hundreds of people during the Troubles—have warned that any move toward Irish unity could rekindle violence. Unionist concerns were not eased when Sinn Fein, the former political wing of the Irish Republican Army and the party most committed to pushing for a border poll in the short term, recently appointed Gerry Adams as its spokesperson on reunification. Adams denies long-standing claims that he was a member and commander of the IRA, which was responsible for most of the killings during the Troubles, and he played a key part in the talks that eventually led to the Good Friday Agreement. But he remains a bogeyman for Northern Ireland unionists. (Adams’s office said he was unavailable to be interviewed for this article.)

If Irish unity seems more likely in the post-Brexit chaos, there is still little sign that it will happen in the short term by orderly means. Though Catholics may soon be Northern Ireland’s largest religious group, they are unlikely to constitute an absolute majority for some time. Nor is it likely that all of them would vote for a change to the present arrangement: After almost a century in existence as a separate statelet, Northern Ireland is showing signs that a distinct identity is beginning to emerge, and that the old sectarian political lines are blurring. In European Parliament elections this May, the nonsectarian and anti-Brexit Alliance Party saw its share of the overall vote rise to 18.5 percent, from 7.1 percent at the last such vote in 2014. Tellingly, the recent Ashcroft poll that found 51 percent of respondents in favor of unity made no allowance for such centrist opinion.

“Society is normalizing, and it’s happening quietly, without any big fuss,” Shirlow said, pointing to studies that show that nearly a quarter of long-term romantic relationships in Northern Ireland are between people from different religious communities, and a third of schoolchildren in the region have a nonreligious background. “Taxi drivers tell me now about how on Sunday mornings they are going to the [Protestant] Shankill Road to pick up kids to go visit granny in [Catholic] Ballymurphy.”

Yet there is nevertheless an urgency to the issue: The consequences of Brexit, in both the short and long term, are unclear. Though the Good Friday Agreement stipulates that the U.K.’s secretary of state for Northern Ireland must call a vote on reunification if he believes there is likely to be a majority for change, in reality the process could be far less predictable and orchestrated. An eventual British withdrawal from the EU could, for example, precipitate Scotland’s seeking, and gaining, independence. Were it to do so, would England, the largest of the U.K.’s constituent nations, want to hold on to distant and troubled Northern Ireland?

“If you have any sense of history,” said Paul Gosling, a Northern Ireland–based English economist who has studied reunification, “you have to recognize that important things can happen quickly and not necessarily in the control of governments.”

If the British government called a “border poll” under the present terms of the Good Friday Agreement, a simple majority would be enough to bring reunification, potentially creating a large, deeply alienated, and possibly violent unionist minority in a newly united Ireland. In light of the bitterness created by the U.K.’s narrow vote for Brexit, some politicians have called for further electoral measures to ensure that such a vote had broad consent, either through a supermajority of, say, 60 percent or through some form of “parallel consent” in Northern Ireland to ensure that both communities were, at least in part, on board with the measure.

But none of these suggestions address the possibility of a chaotic and unintended reunification. The terms of the U.K.’s exit from the EU remain in doubt: The country could leave with a withdrawal deal or without one; on October 31 or months later. It could stay in the EU, and Scotland might leave the U.K. anyway. This uncertainty only feeds fears of what might happen on the island of Ireland.

“Europe and the U.K. are in a muddle, and we could be pitched toward a united Ireland without there being any time to get ready for it,” said Andy Pollak, the former director of the Armagh, Northern Ireland–based think tank Centre for Cross Border Studies. “If it happens that way, it’s going to be messy and angry.”

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