London Editor Frank Millar examines the critical role the Paisley factor will play in the Leeds Castle talks
With what intent is the Rev Ian Paisley attending the negotiations which the Taoiseach, Mr Ahern, and the British Prime Minister, Mr Tony Blair, will join in Leeds Castle this afternoon?
How will he play his strong hand? Will thoughts of his place in history weigh with "the Big Man" after events have conspired to give him the leadership of unionism in the late phase of a stormy and turbulent career?
Or is he splendidly indifferent to what the historians might one day have to say about him?
His advancing years and his most recent stay in hospital have naturally fuelled speculation about the DUP leader's health. Even in the fairly recent past Dr Paisley has been happy to talk about his diet and his weight loss, chortling at those ready to write him off.
Yet he and his family have plainly and understandably been stung by a new tone of irreverence in some discussion of his inevitably failing powers.
For all the public denials, moreover, it seems clear that equally inevitable speculation about the eventual succession has added to the natural tensions in the higher echelons of the collective DUP leadership.
Dr Paisley's family will also be well aware others outside their camp will have mixed feelings about suggestions his grip on unionism may be beginning to weaken.
One leading republican captured that mood last week after watching the DUP leader upbraid the work of "Romanist" journalists.
"I felt sorry for the man," he said, "But not too sorry."
Dr Paisley would pour familiar contempt on such comments. Over the next few days, indeed, he may draw comfort from them.
When did he ever care what his enemies thought of him? Yet this is what tantalises insiders and commentators alike as the newly-configured political representatives of Northern Ireland's two communities enter arguably the most important negotiation since the Belfast Agreement.
Will the Calvinist preacher man play his familiar role as leader of Ulster's "Loyal Opposition", and choose to remain the ultimate "Queen's rebel"?
Or having "defended" his people from all assault, might he think now to claim their tormentors' scalp and lead his own to safety and security?
Mr Blair is plainly right about one thing. A deal between Dr Paisley and the Sinn Féin president, Mr Gerry Adams, would most decidedly "seal the peace".
And the Prime Minister is also right when he suggests that, if the will is there, it should be relatively easy now to accomplish.
For the essential point surely is that the context in which these bitter foes meet and are asked to engage is utterly changed.
Moreover - just as Sinn Féin and the republican movement have changed to a degree which the DUP declines to acknowledge, at least in public - so there are members of the DUP in a state of denial about the extent to which their party also has already changed.
Without question, there are real and substantial issues still to be resolved. There is probably enough in Sinn Féin's outstanding policing agenda alone to reduce Dr Paisley and his colleagues to a state of apoplexy. Yet even before the DUP won its majority unionist mandate to "stop the concession-making to Sinn Féin", this process has been shown to be more than a one-way street.
And in the period following its election victory last November - forced by its leadership responsibility at the very least to play the "Blame Game" - the DUP has already shown a flexibility and imagination which could not be admitted in the days of opposition, when the only purpose was to bring down David Trimble's Ulster Unionists.
Indeed, one principal cause for hope in the Northern Ireland Office and 10 Downing Street is that even then - while publicly campaigning to destroy the Belfast Agreement - the DUP accepted its entitlement and appointed "ministers in opposition" to serve in the powersharing administration established by the agreement.
So on policing, for example - notwithstanding its bitter opposition to the reform of the RUC - the pragmatic DUP might expect to find allies in London at least should it transpire that Sinn Féin has ambitions which go significantly beyond the recommendations of the Patten Commission.
London and Dublin are plainly hoping that, in the same pragmatic spirit, the DUP can allow progress on issues left over from last year's British-Irish Joint Declaration - and the ultimately failed Adams/Trimble negotiation - while awaiting subsequent adjudications on IRA decommissioning and the cessation of all other forms of paramilitary and criminal activity.
DUP sources confirm it would be possible "in principle" to agree such an approach, heralding initial bilateral moves between the British and republicans, pointing toward the creation of a new powersharing Executive perhaps by next March.
Indeed, senior Ulster Unionists are convinced that a readiness for such a "two-stage process" is the only explanation for Dr Paisley's enthusiasm to discover the terms of Mr Trimble's aborted deal with Mr Adams of last October.
Which brings us to the potentially vital distinction between Dr Paisley's position now and Mr Trimble's then. If he chooses to engage, Dr Paisley can do so as the leader of a united party and with every reason to anticipate further electoral reward if the result is a credible and sustainable political settlement.
To which should be added that - contrary to an impression fostered in some government circles and elsewhere - the DUP deputy leader, Mr Peter Robinson, emphatically wants an agreement reached while Dr Paisley is at the helm.
There, however, is the rub for Sinn Féin and the IRA. Whatever notions are entertained about a softer, gentler DUP leadership to follow after Dr Paisley in the years ahead, in the here and now these two seem totally at one as to the terms required of the republican movement.
And, as they will doubtless remind him at Leeds Castle, the DUP's terms are those set out by Mr Blair himself in his "acts of completion" speech in Belfast on October 18th, 2002, when he declared: "The crunch is the crunch. There is no parallel track left. The fork in the road has finally come. Whatever guarantees we need to give that we will implement the Agreement, we will. Whatever commitment to the end we all want to see, of a normalised Northern Ireland, I will make. But we cannot carry on with the IRA half in, half out of this process. Not just because it isn't right any more. It won't work any more."
As Mr Blair observed after his meeting with the Taoiseach last Friday, the agenda hasn't changed, and it can be done if the will is there. The question, however, is whether Mr Blair and Mr Ahern have the will to demand it.