Ireland’s leaders face a serious dilemma. They are pushing hard to ensure the U.K.’s departure from the European Union doesn’t leave it with a visible land border, but they want to avoid alienating the country’s larger neighbor with which it has only lately become reconciled.
As fellow EU members, the U.K. and Ireland trade without customs checks and the two countries have an agreement to allow people to freely cross their borders. Since the 1998 Good Friday peace agreement, security checkpoints have been removed between Ireland and Northern Ireland, which is part of the U.K.
Both sides have pledged that there will be no resurrection of a physical land border after Brexit. Yet British Prime Minister Theresa May has categorically ruled out the one concrete solution so far to have emerged: An EU proposal that would, in effect, have Northern Ireland remain a part of the EU’s customs union and single market, which Mrs. May has said the U.K. must leave.
Ireland has influence over Brexit that belies its size. EU and British negotiators agreed Monday on a 21-month transition for the U.K. after it leaves the bloc in March 2019. However, Ireland effectively secured a veto over the overall Brexit deal from other EU members should it be unconvinced by the U.K.’s border proposals. Yet wielding the veto over the border issue would stir British animosity.
From the moment British voters chose to leave the EU in June 2016, Ireland’s government has sought to ensure that the Brexit negotiations result in “the closest possible relationship” between the U.K. and the EU. Mrs. May’s stance on the customs union and single market makes that a challenge.
At an economic level, Ireland can see little upside from Brexit, and only a range of more or less bad outcomes. Ireland’s Economic and Social Research Institute on Wednesday estimated that a “hard” Brexit that led to new tariffs would increase the cost of living for households by between 2% and 3.1%, with those on lower incomes suffering most.
But the potential damage goes beyond economics.
“The relationship with the U.K. will inevitably be set back,” said Daithi O’Ceallaigh, a former Irish ambassador to the U.K., and chair of the U.K. Group at Dublin’s Institute of International and European Affairs. “It developed when both were members of the EU, and the relationship will not be as close.”
The two countries joined the EU at the same time, and Ireland was marginally the less enthusiastic of the two, with conservatives fearing membership of the bloc would dilute an idiosyncratic culture built around the rejection of modernity and a fierce attachment to Catholic doctrine.
They were right. The big surprise was how joint membership of the bloc helped bring an end to old antagonisms between the two countries. Turning its gaze to the rest of Europe helped Ireland grow out of its obsession with perceived mistreatment by Britain, and by the time Irish voters were asked to renounce a territorial claim on the part of the island that was in the U.K. in 1998, a staggering 94% assented.
But progress has been slow, and it wasn’t until 2011 that Queen Elizabeth II made the first official visit by a British monarch to Ireland in a century. The first official visit by an Irish president to the U.K. followed two years later.
In a speech at Windsor Castle during that trip, President Michael D. Higgins quoted an Irish language phrase: “Ar scáth a chéile a mhairimid.” That roughly translates as “We live in the shadow of each other.”
That is the way it still is, and the way it will be after Brexit.
Some Brexit advocates have already declared their unhappiness with Irish Prime Minister Leo Varadkar’s handling of the negotiations, accusing him of playing what is known in Ireland as the “green” card to avoid losing support to the nationalist Sinn Féin party.
They include Arlene Foster, the head of Northern Ireland’s pro-British Democratic Unionist Party, whose support is essential to Prime Minister Theresa May’s minority government.
“He wants to look as if he is strong, particularly in relation to the Brexit negotiation,” she said. But, she said, “if Brexit is to work, he has to understand he has to have a working relationship with those of us in Northern Ireland who see ourselves as British.”
Mr. Varadkar denies charges that he is attempting win votes by taking a nationalist stance, or seeking to advance a united Ireland by stealth. His Fine Gael party hasn’t historically been in a contest for the same voters as Sinn Féin, since they come from opposing ends of the Irish political spectrum.
But the fact is that the most sensitive interaction between Irish and British governments concerns their joint custodianship of the 1998 agreement that ended the conflict in Northern Ireland. Avoiding a border at the price of alienating its hard-won partner in that endeavor isn’t an outcome the Irish government would relish.
—Jason Douglas contributed to this article.
Write to Paul Hannon at paul.hannon@wsj.com