The IRA's failure to include commitments by Mr Gerry Adams as part of its formal position during the most recent phase of the North talks had halted progress towards elections this month, the Taoiseach has confirmed.
Efforts to hold the Northern elections in late May could have continued if the IRA had confirmed earlier that the Sinn Féin president spoke for it, Mr Ahern said yesterday.
During the talks late last month Mr Adams said the IRA was prepared to put arms beyond use, end the conflict and disengage from actions that would undermine the peace process.
However, the Taoiseach said he had sought "in the last call that I made" that the IRA would agree with this interpretation: "And I was told, 'No'. I was told that there was no possibility of that."
The IRA's reluctance to include Mr Adams's commitments as part of its position was reported by The Irish Times last Saturday.
Mr Adams later said the report was "quite untruthful".
Mr Ahern pointed out yesterday that the IRA had since shifted its position.
This week's additional IRA statement, in which it said Mr Adams had "accurately reflected" its position, had helped greatly. "There was obviously a seismic shift in six days. It is welcome," he said.
He said he believed that the IRA and Sinn Féin had softened their positions because the Government "had put the question up to them" and because of poor public and media reaction.
Though republicans have still to satisfy Dublin and London that the conflict is finally over, Mr Ahern said the Ulster Unionist Party also had questions to answer.
"We need to hear David Trimble and all of his party clearly state that they want to see cross-party co-operation in the Assembly and that they are prepared to work this," he said.
The "nonsense of the last four years created in many cases by him and his colleagues of not working the institutions has to stop", he said.
"If we get clarity on one side, we also need to get it on the other side," he said.
However, Mr Trimble said there was "no moral equivalence" between violent republicanism and unionism. "Nationalists know that the real problem is the failure of republicans to stop all paramilitary activity," he said.
"They are keen to point out that this is the essential stumbling block but for reasons of communal solidarity, they feel that if they're going to bash republicans, they also have to have a go at us," he said.
Asking if Mr Ahern was "living on another planet", Ulster Unionist MP the Rev Martin Smyth said the Taoiseach's remarks "clearly" indicated that he would have preferred the Assembly to operate with Sinn Féin/IRA acting as they pleased.
SDLP leader Mr Mark Durkan commended Mr Ahern, saying: "It is not enough for David Trimble to make demands of the republican movement. He also needs to make clear what he will do to comply with the agreement.
"That means working all of the institutions including the power- sharing executive and the North-South bodies," said Mr Durkan, who will attend the Labour Party's Killarney conference this weekend.
Hinting that he would offer reassurances about the IRA's future existence next week, Mr Adams appealed to ordinary unionists to do what they could to save the peace process.
"We still need to work with unionism and I'm going to make some remarks next week which I intend to be an attempt to express republican concerns about unionist fears and sensitivities around all these matters.
"I would appeal almost over the head of their political leaders to unionists to understand that we're now in a period of total political uncertainty," he said, adding that the peace process now has "no centre of gravity".
Welcoming the dismantling of two South Armagh watchtowers, the Taoiseach said it showed that London still retained "faith" that progress can be made in coming weeks.