Prime Minister Johnson huddled with Northern Ireland’s five largest parties on Monday under the storm clouds of a political crisis that could yet be felt not only in those six counties but also England, the Republic of Ireland, and all of Europe. 

Mr. Johnson is mulling altering the Northern Ireland Protocol, the set of rules governing the post-Brexit customs and immigration issues between England, Northern Ireland, and, via the Irish Republic, the European Union. If he does so, many warn that it could precipitate a trade war with the European Union.

The British prime minister arrived in Northern Ireland during a standoff between the republican nationalist party, Sinn Féin, whose motto reads Tiocfaidh ár lá, or “our day will come” — referring to Irish independence — and the Democratic Unionist Party, ardent loyalists to Elizabeth II.   

The Protocol, a byzantine series of workarounds, is a flashpoint for unionists in Northern Ireland, who see in its customs regulations a barrier between it and Britain — a betrayal of both the spirit of Brexit and their hope for a frictionless convergence with England. For Sinn Féin, altering it is reckless and misguided.   

Reports out of Westminster indicate that Mr. Johnson’s foreign secretary, Liz Truss, is set to introduce legislation this week to unilaterally alter the Protocol in the name of “internal peace and security,” telegraphing concerns over anti-Protocol sentiment in Northern Ireland.

Mary Lou McDonald, president of Sinn Féin, has accused Mr. Johnson of using Northern Ireland as a “pawn” in service of his larger struggle against the EU, and acting in “cahoots” with the DUP.    

As a whole, 56 percent of Northern Ireland voted against Brexit. That number rose to 66 percent among unionist parties like the DUP, and cratered to 12 percent among the nationalist camp, spearheaded by Sinn Féin.     

The Foreign Office reported that the EU’s Brexit negotiator, Maroš Šefčovič, was less than receptive to any changes. He expressed the opinion that there was “no room to expand the negotiating mandate or introduce new proposals to reduce the overall level of trade friction.”    

As Mr. Johnson wrote in the Belfast Telegraph, dissatisfaction with the Protocol is widespread. “Every unionist representative campaigned against the Protocol, as currently constituted. More importantly, every party, across the divide, seeks mitigations and change.”

As the prime minister searches for “a sensible landing spot” of a revised Protocol in the face of EU opposition to renovating the 2020 agreement, the continental future of the Protocol is now tied to Northern Ireland’s internal politics, which have themselves entered a period of upheaval.       

In elections last week, Sinn Féin won the most seats in Northern Ireland’s assembly, the Stormont. This political thunderclap — or volcano — raised the prospect of a new chapter in the history of Ireland, and the United Kingdom. 

Sinn Féin won 27 seats, to 25 allotted to the incumbent Democratic Unionist Party. In the wake of that victory, Mrs. McDonald said “the preparation for constitutional change in Ireland needs to begin now,” adding “that “change is under way.” Presumably, this change is intended to be toward a united Ireland.    

Not so fast, though. The DUP has refused to nominate a speaker for the Stormont, effectively paralyzing the assembly. Under power-sharing rules, the assembly and executive cannot function without the DUP. What is called a “zombie” government has ensued.  

The leader of Sinn Féin in Northern Ireland, Michelle O’Neal, accused the DUP of “disgracefully holding the public to ransom for their Brexit mess.” For his part, the leader of the DUP, Sir Jeffrey Donaldson, explained pulling the plug on the Assembly by noting that “the long shadow of the Protocol is casting its mark over this place.”

Behind the disputations over the Protocol is an even larger concern — that a metastasizing crisis over the relationship between England, Northern Ireland, and the European Union could threaten the 1998 Good Friday Agreement.

That accord ended the violence between unionists and republicans known as “The Troubles” and brokered a power sharing agreement in Northern Ireland. The DUP was the only party in Northern Ireland not to sign on. 

Among other terms, the Good Friday Agreement laid out a pathway toward a united Ireland when it appears likely 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.”

In that case, the secretary of state is empowered to call for a border poll to determine whether Northern Ireland wishes to join the Republic of Ireland to its south.   

The current secretary, Brandon Lewis, a Conservative, acknowledged that the vote was a “significant moment for Northern Ireland” but appeared to refrain from endorsing a border vote, instead changing the subject to bread and butter issues.

Despite those intentions, the emerging crisis over the Protocol, coupled with the triumph of Sinn Féin in Northern Ireland, suggests that core questions of border and identity will continue to define the political scene on the Emerald Isle.