Unionist apprehensions soared last night with the confirmation that the British Prime Minister, Mr Tony Blair, was on his way back to Hillsborough.
The Taoiseach, Mr Ahern, had earlier in the afternoon indicated his plans for a swift return. But it was only when Downing Street confirmed that Mr Blair was on his way they knew, with dread certainty, that the game was on.
Mr David Trimble and his Assembly team had departed the scene earlier in cheerful and confident mood, having unilaterally announced the effective adjournment of the talks process for two weeks.
They had not, it seemed, reckoned on Mr Ahern, and his determination that any move to adjourn or "park" the process could ultimately imperil the Belfast Agreement.
Mr Blair had seemingly left himself a get-out clause, suggesting he would rejoin the talks only if sufficient progress was made to warrant further time out from a fully fledged Kosovan crisis.
That factor alone weighed heavily in the assessment of the seriousness of prime ministerial intent as the negotiations were resumed. As one British source put it: "You don't get the Prime Minister coming here to sit up all night, in the middle of a war, to deal with frivolity."
And there was nothing frivolous about the Irish/British proposals placed before the parties. After all the grandstanding, speculation, buck-passing and blame allocation, suddenly we were down to the fundamentals and real hard choices.
At the wholly shambolic conclusion to the morning session, Mr Ahern had intrigued with the assertion that the questions were now of timing and dates: "I think the principles are established."
But if one leading nationalist player thought the Taoiseach's assessment a touch "simplified", the emerging plan made it clear that agreement on the way forward must be rooted in the principle that decommissioning is, after all, an "obligation" under the terms of the Belfast Agreement, and must be completed by May 2000.
It is claimed that Mr Gerry Adams and Mr Martin McGuinness had accepted as much on Tuesday night, although Mr Blair apparently let it be known that a Sinn Fein promise on this issue would not be an acceptable substitute for an IRA commitment.
The acceptance by the republican movement of the principle - that decommissioning is required by the agreement - has always been central to the viability of Mr Seamus Mallon's attempts to move the Ulster Unionists and Sinn Fein from their "absolutist positions".
On the assumption, if not the understanding, that that commitment would be forthcoming, Mr Trimble was last night being asked to agree the immediate formation of the Northern Ireland executive in "shadow" form.
Within a month thereafter the IRA would be required to put some weaponry "beyond use" in a "voluntary" process to be acceptable to, and verified by, Gen de Chastelain's International Commission.
And consequent upon that, the British government would formally transfer power to the Northern Ireland Assembly and its executive.
Mr Trimble, accompanied by his entire Assembly party, made his way to Hillsborough determined to secure fundamental changes to this plan, unsure, even then, if it would prove acceptable to his colleagues.
For the deficiencies of the proposal, from an Ulster Unionist perspective, were immediately apparent.
Nowhere did the two-page document apparently require an actual act of physical decommissioning, the "event" long insisted upon by Mr Trimble.
Nor did it define the means of establishing an agreed inventory of the weaponry in the IRA's possession, or prescribe a timetable for its destruction.
Against that, Mr Trimble must have felt himself confronted with a tantalising prize. Just weeks before, his security spokesman, Mr Ken Maginnis, had said that "the certainty of achievement" mattered more to his party than the modalities.
Now, seemingly, he was faced with the proposition that the republican movement - which had strenuously rejected the idea that it faced any requirement to decommission - might be prepared to bow before the weight of public opinion which Mr Trimble had skilfully nurtured and led since last summer.
Sensing Mr Blair's determination that there would never be a better time to secure the Belfast Agreement, Mr Trimble - the academic lawyer - faced the daunting prospect of testing the veracity and reliability of every assurance offered, while pushing for further concessions from a Sinn Fein leadership which suddenly appeared to have seized the tactical initiative.
Even if convinced himself, Mr Trimble the politician - who had already expended much political capital, gaining in the process a reputation for drawing lines in the sand only to see them washed away - faced the still-more daunting task of persuading his party that this represented the final, necessary and justified "risk" for peace.
His first task would be to convince himself. His second, to convince already distrustful and suspicious colleagues. Could he? Would he?
At this writing - and with a long night of negotiation ahead - it was impossible to predict.
There has been detectable in Irish circles in recent weeks a certain comfortable assumption that Mr Trimble and his colleagues were the most anxious to preserve the Belfast Agreement.
But those who know him well believe the Ulster Unionist leader perfectly capable of walking away - incurring Mr Blair's wrath if need be - if persuaded the deal is wrong for him and his party.
Only two things could be said with certainty. Seldom can an Ulster Unionist leader have felt quite so alone. And not since the events of 1974 can any of his successors have been so acutely conscious of the shadow of Brian Faulkner.