The brutal killing of Denis Donaldson, the informer who thought he had got away with it, has disrupted the Irish peace process and overshadowed today's attempt by Tony Blair and Bertie Ahern to get politics moving again. If it turns out that the IRA ordered his assassination, or endorsed it, then the hearts of all those who want to see the process underpinned by a new political architecture will sink into their boots.

In recent years momentum has ebbed away from the Northern Ireland process. Once the world took note of events in Belfast: more recently most of the world, governments excepted, have switched off.

It has taken the spilling of blood on the floor of a Donegal cottage to re-engage wider attention - the Donaldson killing provides graphic confirmation that violence grips the interest while politics very often does not.

The peace process has delivered the huge prize of a drastic reduction in funerals. Murders such as the death of Donaldson are now a rarity rather than the daily constant they once were.

Last year the IRA declared that it was going away, had decommissioned its entire form-idable armoury and was committed to the idea of politics. Extreme Protestant groups are still active but their death toll too has been greatly reduced.

Yet many problems remain; it is taking years, and causing continuing pain, to clear the battlefield. Terrorism has in many ways mutated into criminality so that, while killings may be uncommon, paramilitary groups still maintain illicit funds and sometimes try to add to them.

The other central problem, which Blair and Ahern are to address today, is that Northern Ireland has no government of its own and continues to be run through direct rule from Westminster. This is partly due to an earlier stage of the Donaldson saga, when in 2000 the revelation of a republican political spy-ring caused the First Minister, David Trimble, to pull the plug so that the Stormont Assembly shut down.

Since then, politics has been idling in the doldrums, with the falling death rate not matched by constructive activity. In fact, the reduction in violence has caused many, particularly in the Protestant community, to conclude that the absence of power-sharing is no great loss.

The absence of devolution is not merely an academic abstraction. The IRA and Sinn Fein sold the peace process to their grassroots on the basis that the gun was no longer necessary since they could wield more clout through politics.

While there is no suggestion of any return to full-scale war, the question hangs in the air: if there is no meaningful political activity in Northern Ireland, will the republican grassroots eventually come to conclude they have been taken for a ride?

This, and the feeling that local parties should be learning to work together, is why Blair and Ahern have been toiling to put the Assembly back together. Most local parties, and certainly Sinn Fein, share their goal.

The two prime ministers want to coax everyone back into the Assembly with the hope that later in the year agreement will be reached and a new powersharing coalition can emerge.

The big uncertainty about all this centres on the Rev Ian Paisley, whose Democratic Unionist party has in recent times trounced Mr Trimble's outfit to become the primary Protestant political voice. Unionism is in his hands.

Paisley is a devolutionist but - even though his 80th birthday falls today - he is by no means a man in a hurry. Making a deal would crown his lengthy political career with the office of First Minister, a position he would enjoy hugely.

However, that could only happen if he makes an accommodation with republicans: in fact it would entail someone like Martin McGuinness as his deputy. The fact that this is even a possibility is an indication of how far the peace process has come.

But Paisley has made it abundantly clear that he will not make an arrangement with Sinn Fein until he is absolutely certain that the IRA has gone away once and for all. He says it has to give up not just violence but anything that could be classed as criminality.

No one is really sure whether this stance is genuine or merely a device to play for time, but the official hope is that by autumn official monitoring reports will give the IRA a clean bill of health. But this is where the Donaldson murder comes in.

Paisley will not move unless and until it is shown that the IRA was not involved in the killing. So long as there is any shadow of doubt about this he will not go into government with Sinn Fein, there will be no devolution, and the peace process will remain a valuable but incomplete enterprise.