The Northern Ireland peace process is often described as a rollercoaster ride and this week it has been in one of its downward loops.

There was never any real expectation of a major breakthrough when most of the province's political establishment camped out on the White House lawn as part of the St Patrick's Day celebrations, but it had been hoped the more rarefied climes of the United States might have helped nudge the process forward.

Events at home however, appear to have removed what little room for manoeuvre there was.

IRA decommissioning unlikely

The murder of prominent Catholic lawyer Rosemary Nelson not only plunged her family into despair, it also had the effect of making it even more difficult for the IRA to countenance any form of decommissioning.

Loyalists were now killing nationalists again and republicans consider themselves to be defenders of their own community.

If it emerges elements of loyalist paramilitary groups currently on ceasefire were involved in the Rosemary Nelson murder, that will harden the IRA line on a weapons handover.

At the heart of the current impasse are the mutually exclusive positions of Ulster Unionist leader David Trimble and Sinn Fein president Gerry Adams.

Mr Trimble is insistent the IRA must begin to dispose of its weapons before Sinn Fein can take ministerial seats in a new Northern Ireland government. He argues his party would dump him if he relented.

New deadline

But Mr Adams says the demand for arms is a precondition, is not in the Good Friday Agreement, and he simply cannot deliver the IRA on this issue.

He argues the IRA would split if his hand is forced. The British and Irish governments, backed by the Clinton administration, are deeply worried the whole peace process could unravel over the arms question and are exasperated by the amount of megaphone diplomacy between the two sides.

There is a compromise position being floated which might be called "simultaneous blinking", where the IRA agrees to a starting date for decommissioning and Mr Trimble allows Sinn Fein to take their ministerial positions in the Northern Ireland Assembly in the interim, but so far neither side is ready to blink unless the other blinks first.

A deadline of Good Friday (just like last year) has been set for a resolution of the impasse and the devolution of powers to the new Assembly, but at this stage there are no signs the deadline will be met.

The British and Irish prime ministers are poised for another diet of eat, sleep, negotiate in Belfast but the real focus is on David Trimble and Gerry Adams.

Unionist support faltering

They hold the key to the future, a key they must both turn at the same time if this is all to work.

If there is no deal by Good Friday, Northern Ireland faces an uncertain summer. The Good Friday Agreement will have to be "parked" for a review which might not properly get underway until September, after the marching season which has the capacity to destabilise everything.

Even before the parades get into full swing there is the European election campaign which is already starting.

The DUP leader Ian Paisley is promising to turn it into another referendum like that which ratified the Good Friday Agreement last May.

A recent poll for BBC Northern Ireland's Hearts and Minds programme suggests a clear majority of Unionists would now vote against the Agreement if given the chance.

Everyone in the pro-agreement camp is desperate to find a deal which will avert a long, hot and potentially dangerous summer. But at the heart of this current dispute over decommissioning is lack of trust.