The breaking of the logjam at Stormont, Northern Ireland’s devolved parliament, puts a united Ireland — a long-deferred dream for some, and a dreaded nightmare for others — closer to consummation than it has been since the founding of the Republic.

Sinn Féin’s leader at Dublin, Mary Lou McDonald, marked the occasion by observing to the press at Stormont that a unified Ireland is now within “touching distance” and that the era of partition is “now gone; it is now consigned to the pages of the history books.” She heralds a “new constitutional dispensation ending partition.”

Ms. McDonald evokes a “new Ireland” and a “historic turning of the wheel” that “signifies that what we talk about” — a united Ireland free of British rule — “now is possible.” The end of partition would mean the joining of the six counties of Northern Ireland with the 26 ones of the south, undoing a status quo of division that took shape in 1921.  

While the separation between Ireland and Northern Ireland has held for a century — and is not guaranteed to end anytime soon — there is a mechanism for reunification. An amendment to the Irish constitution passed in the wake of the Good Friday Agreement of 1999 “recognises that a united Ireland shall be brought about only by peaceful means with the consent of a majority of the people, democratically expressed, in both jurisdictions in the island.”

A statute of parliament ordains that the secretary of state for Northern Ireland, who sits at Westminster, “shall exercise the power [to hold a referendum] if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland.”

That possibility came into focus when Sinn Féin pulled off a shocking win in the last election. It was the first time that the republican and democratic socialist party, the political heirs to the Irish Republican Army and fierce partisans of a united Ireland, garnered the most seats at Stormont, the traditional stronghold of Protestant and unionist sentiment. Northern Ireland’s next first minister, Michelle O’Neill, is a member of Sinn Féin.

Ms. O’Neill’s historic victory is not news — she notched it two years ago but is only now set to claim the mantle of leadership after a  deal between the pro-British Democratic Unionist Party and London that will restore a functioning government to the six counties that are still loyal to the Crown. The DUP has been boycotting the devolved parliament in Northern Ireland. 

The sticking point for the leader of the DUP, Jeffrey Donaldson, has been post-Brexit trade rules. Northern Ireland voted to “Remain,” though the DUP took a strong pro-Brexit position. Mr. Donaldson opposed the Northern Ireland Protocol negotiated by Prime Minister Johnson to govern trade between Ireland, Northern Ireland, and Britain. It could be, though, that Mr. Donaldson’s obstreperousness was downstream from a feeling of “The World Turned Upside Down” and the shock of losing to his foes.    

Mr. Donaldson’s party also set itself against the Windsor Framework, an update to that agreement signed by Prime Minister Sunak. A sticking point for the DUP was the effective border of the Irish Sea, which kept Northern Ireland in the European Union’s single market for goods, even after Brexit. 

Now, though, Mr. Donaldson declares that “we are prepared to move forward.” The British Broadcasting Company reports that the deal “will reduce checks and paperwork on goods moving from Great Britain to Northern Ireland.” The accord likely comes as a relief to Mr. Sunak, who called it a “decisive breakthrough” and a harbinger of a “brighter future.”

Sinn Féin hopes that the future includes a victory in Ireland’s next elections, in March 2025. The party now sits in the opposition, as the republic is governed by a three-party coalition. While Sinn Féin has long waxed in the polls, setting the stage for it to be in power on every inch of the island, some recent slippage could serve as an antidote for electoral hubris. 

According to polling done by the Business Post, “support for Sinn Féin has slumped to its lowest level since the last general election,” in 2020. Ms. McDonald’s party has dropped four points in just two months, an interval that overlaps with growing concern — and at Dublin, riots — over the issue of immigration.

A closer storm is brewing, though, for Ms. McDonald with respect to that signature Irish holiday, St. Patrick’s Day. Some in her party, which is staunchly aligned with the cause of the Palestinian Arabs, whose wars against Israel are seen to reflect Ireland’s struggle against the British, are calling for her to boycott the traditional visit to the White House to protest President Biden’s support for the Jewish state. 

Ms. McDonald, though, plans to go, even as she on Friday at a town hall described her country’s relationship with America as “complicated” because Washington is “very much on the wrong side in respect of Palestine.” While Ireland has long seen itself as a standard bearer of the worldwide anti-Israel coalition, it also enjoys close ties with America. A boycott of the St. Patrick’s Day festivities — hosted by an Irish-American president, to boot — could redound negatively for Dublin.        

Sinn Féin’s current leader cites its past president, Gerry Adams — Ms. McDonald calls him ​​“our friend, our inspiration, our leader” — for the position that “what is important for us in the USA is Irish-America,” meaning that she should go. He explains that “serious people involved in struggle” understand that such a break with America over Israel “would set back our own struggle.” 

Mr. Adams, whose membership in the Irish Republican Army at the height of the Troubles has long been a matter of debate, at that event on Friday called Sinn Féin “the second cousin of the army and it was armed actions which set the pace” during the long years of conflict. The party’s name comes from the Gaelic Sinn Féin Amháin, or “ourselves alone.”