Pro and anti-agreement unionists are signalling that a British/IRA deal on "acts of completion" could trigger a formal split in the Ulster Unionist Party.
Leading dissidents intend to force Mr David Trimble to put the terms of any agreement to a special meeting of the party's ruling Ulster Unionist Council.
This was confirmed last night on the eve of tomorrow's annual meeting of the UUC, and as the Taoiseach, Mr Ahern, and the British Prime Minister, Mr Tony Blair, reviewed preparations for next Monday's crucial talks with the pro-agreement parties in Belfast.
Speaking after an hour-long meeting with Mr Blair at Downing Street, Mr Ahern said Monday would be "an important day" but that he could not yet predict a successful outcome to the talks.
The negotiations are designed to secure the cessation of the IRA as an active paramilitary organisation, the restoration of devolution, and reciprocal British acts to complete the implementation of the Belfast Agreement.
Mr Trimble's current position appears to be that emergency meetings of his party's ruling body will be unnecessary, since the expected IRA offer will almost certainly fall short of what he believes necessary to command widespread unionist confidence.
However, senior pro-agreement figures are privately speculating that convincing moves by the IRA towards disarmament and the effective standing-down of its paramilitary capability could see the existing divisions within the UUP formalised.
Such predictions of possible political convulsions ahead underline the fragile nature of the present UUP truce, and would seem to confirm that - against their publicly stated expectations - at least some senior figures in the Trimble camp allow that the present negotiation could have a successful outcome.
A major complicating factor in the ongoing negotiation has been Sinn Féin's doubts about both Mr Trimble's intentions and his capacity to deliver on any agreement to resume power-sharing.
Against that, Downing Street has been sustained by the belief that the majority of Mr Trimble's Assembly Party calculate that the successful restoration of devolution would provide them with the best possible basis on which to fight off the DUP challenge in the Assembly elections scheduled for May 1st.
Mr Trimble has not decided he could fight the election only from outside the Executive.
However, as British and Irish officials prepare to work through the weekend, it seems clear they have yet to resolve a number of the UUP leader's key demands.
Apart from the specifics of any IRA statement redefining and extending the terms of its cessation of military operations, it is understood Mr Trimble is still demanding a significant independent element in the proposed arrangements for the monitoring of paramilitary ceasefires.
New sanctions against any party subsequently deemed in breach of its commitment to exclusively peaceful and democratic means are also being sought, it is understood.
Usually reliable sources suggest that without independent monitoring and sanctions "with a degree of automaticity", Mr Trimble is unlikely to feel able to endorse any package agreed between the British government and the republican movement.
Official sources admit it is proving difficult to square the circle between Mr Trimble's demands and Sinn Féin's insistence that the institutions of government be protected against further unionist attack.
Unionist hardliners are also predicting that, even if next Monday's talks produce a breakthrough, Mr Trimble could still be forced to seek a postponement of the May elections to sell a new agreement to unionist voters.
While the British government's internal debate about election options appears unresolved, the Taoiseach yesterday again signalled his determination that the election should proceed come what may.
Asked if there was a hard and fast commitment to the scheduled election date, Mr Ahern replied: "May 1st is the date under law for an election."
Challenged that the law could be changed, the Taoiseach said: "It can't be changed unless that's changed within a few days. I haven't heard anyone suggesting a change."