By many measures, Northern Ireland has made remarkable progress since the formal end in 1998 of three decades of conflict between nationalists, drawn mainly from Catholic backgrounds, who want to unite Northern Ireland with the Republic of Ireland; unionists, drawn mainly from Protestant backgrounds, who want to remain part of Britain; and the British state itself.

The region has mostly been governed, through a power-sharing agreement, by a coalition of nationalists and unionists. The region’s main militant force, the Provisional Irish Republican Army, gave up its weapons in 2005. And border crossings between Northern Ireland and the Republic of Ireland have been removed, making it easier for people on either side to live lives that span both territories.

Yet in recent years Northern Ireland has begun to drift.

The power-sharing agreement has collapsed because of disagreements between Ms. Foster’s and Ms. McDonald’s respective parties. For more than two years, the impasse has left Northern Ireland in the hands of civil servants, who have avoided making significant political and financial decisions.

The region is still heavily segregated along sectarian lines.

Border checks may return if Britain leaves the European Union without signing an agreement about its future relationship with the bloc.

And an I.R.A. offshoot, known in the news media as the New Irish Republican Army, has been linked to several terrorist attacks, including a car bomb in Londonderry in January — and last week’s shooting.