Mr David Trimble will give the Taoiseach, Mr Ahern, no guarantees about an early Ulster Unionist return to Stormont when they meet in London this morning.
This became clear last night ahead of a Downing Street summit between Mr Ahern and the British Prime Minister, Mr Tony Blair, this afternoon, which Sinn Féin has described as potentially the most crucial meeting on Northern Ireland of the past 20 years.
British sources appear agreed this claim will prove justified if there is agreement in the present negotiations on IRA and reciprocal British "acts of completion", enabling the reinstatement of the suspended power-sharing Executive ahead of fresh Assembly elections scheduled for May 1st.
At their prior meeting in the Irish Embassy this morning, Mr Trimble is expected to tell the Taoiseach that his mind "is not closed" to such a possibility, despite the fresh UUP divisions which would almost certainly be triggered by any decision to resume power-sharing with Sinn Féin ahead of an election.
However, authoritative sources have told The Irish Times that, despite the apparent enthusiasm of some of his Assembly colleagues for a deal enabling them to campaign on the "success" of devolution, Mr Trimble himself is "deeply sceptical" about whether any credible agreement can be concluded in the time-frame currently envisaged by both governments.
At the same time Mr Trimble is thought likely to leave the Taoiseach in no doubt that, while he has followed Mr Blair in declining to use the so-called "D-word", unionist demands in respect of decommissioning and the cessation of all forms of paramilitary activity amount to an effective call for the disbandment of the IRA.
Unionist strategists suggest that Mr Trimble's non-use of the D-word is largely a matter of tactics to avoid subsequent demands from sceptics within his party that he prove disbandment has in fact taken place. However, the sources said: "But that effectively is what we mean. The private army must be completely out of the picture and no longer there."
During his private pre-Christmas briefing of senior British journalists in London, the Taoiseach made it clear that any significant statement by the IRA about its future was likely to fall short of disbandment, and recent speculation has centred on the possibility that it might announce an intention to "stand down" units.
Ahead of today's summit meeting, however, the Ulster Unionists are saying this would be "nowhere near the ball park" in terms of a package of "acts of completion" which Mr Trimble could hopefully take back to his party's ruling Ulster Unionist Council.
Last night attention was being drawn to the resolution finally passed by the UUC last October saying "it must be demonstrably clear that the transition (from terror to democracy) is coming to an end", and to the fact that this critical amendment was drafted by Mr Trimble and the former minister Sir Reg Empey.
Insisting that any deal emerging from the present negotiations "must clearly be a winner and not another fudge", the UUP sources suggested they would otherwise prefer to fight the election - assuming it goes ahead - from outside the Executive.
It also emerged this was the position agreed by a key UUP election strategy group, comprising senior representatives of both wings of the party, when it met under Mr Trimble's chairmanship in Belfast last Friday. Confirming this the sources said: "If the deal is debatable, if any questions can be raised about it, the unionist electorate - looking at it against the background of being consistently let down - will assume that any doubt is justified and will work against them. That is the position we agreed."
Irish sources last night suggested there would probably be no communiqué after today's talks over lunch between Mr Ahern and Mr Blair, and that officials might need several days more to agree a framework approach for the negotiation expected to continue for another four weeks.