As a result, the power-sharing mechanism has been plagued by crises from the outset. The question at the heart of the latest political convulsion is whether the Irish Republican Army, the paramilitary arm of the nationalist party Sinn Fein, ever really exited the stage. Or does it lurk in the wings, ready to reprise its role if the political wing cannot deliver?

On Aug. 12, a former I.R.A. member named Kevin McGuigan was murdered in Belfast. Northern Ireland’s police chief, George Hamilton, said he believed that the I.R.A. itself — supposedly disbanded after the Good Friday Agreement — was involved in the killing, even if the assassination had not been sanctioned by its leadership. Although it poses no threat to Britain’s national security, he explained, the I.R.A. continues to exist.

No one in Northern Ireland doubts that the McGuigan killing was in retaliation for the murder three months earlier of a former Belfast commander of the Provisionals, or “Provos,” as the I.R.A. is commonly known. But the effect of this score-settling among Republicans has been incendiary.

Chief Constable Hamilton’s statements were enough to throw the administration into disarray, with the Democratic Unionist Party threatening to pull out of the Executive. To the Democratic Unionist leader, Peter Robinson, the notion that their main partners in the government of Northern Ireland, Sinn Fein, secretly maintained a paramilitary capability was unacceptable. If the unionists followed through on a withdrawal from power-sharing, the entire edifice of the Good Friday Agreement would collapse — possibly forcing Prime Minister David Cameron to restore direct rule from Westminster.

Cynical observers reckon that such a calamity is unlikely. They argue that Democratic Unionist Party and Sinn Fein politicians have equally come to love their sinecures and won’t easily give them up. Their argument is strengthened by the fact that, despite all, the Democratic Unionist Party continues to attend all-party talks on a way out of the difficulties. What’s certain is that the latest impasse is the most fraught so far.