THIS WAS TO be the week that the pieces of peace fell together in Northern Ireland, and so they did. On cue, hitherto hostile parties of the Protestant majority and Catholic minority in the British province formed a government together. Britain devolved important powers of local self-government upon it. The Irish Republic surrendered its constitutional territorial claim on Northern Ireland. It was a fine week for the Irish in their several communities, and a fine week for Britain, too.

But since there is a long history and much residual mistrust, changing the institutional furniture is not the end of the story. The violent faction of Irish nationalists, whose program calls for reuniting Northern Ireland with Ireland proper, remains to be brought aboard. The Irish Republican Army, or IRA, this week met a key term of the peace accords by assigning a representative to meet with the international commission set up to oversee the disarming of Northern Ireland's paramilitaries. But David Trimble, the Protestant leader of the new joint government of the province, is under compulsion by his party to quit that government if the actual disposal of weapons has not begun by Feb. 1. So far the IRA has not turned in a single grenade, nor has it said it will.

So let us celebrate first of all the politicians on both the Protestant and Catholic sides who risked their political fortunes and more to rescue their Northern Ireland from a quarter-century's sectarian strife in which some 3,000 people died; the British and Irish governments played crucial supporting roles. And let a generous bow then be made to President Clinton for converting what at times seemed to be a careless political intervention into an essential and successful diplomatic initiative; his agent, George Mitchell, performed prodigies of mediation.

For final cheers, however, let us wait a few months more for the onset of disarmament of the paramilitaries, especially the IRA. It has made no promises and apparently still has not renounced the idea of armed struggle, although, thankfully, almost everyone else in Northern Ireland has.