With the Mitchell review of the Belfast Agreement entering a crucial week, the Sinn Fein president, Mr Gerry Adams, has stated that the gap between his party and the Ulster Unionists may be too wide to bridge.
It would be a "huge tragedy" if the review failed, but this was another reality that had to be faced, he said.
The review negotiations, aimed at solving the impasse over decommissioning and the formation of an executive, will resume early this week in London at an undisclosed location, thought not to be Lancaster House, as Senator Mitchell continues to shield the parties from the media.
At the annual Fianna Fail Wolfe Tone commemoration at Bodens town in Co Kildare, the Taoiseach, Mr Ahern, praised "the courageous political leadership given in recent years by many in Sinn Fein and the loyalist parties".
He said "people should not be so begrudging" about the paramilitary ceasefires, even if they were imperfect. The peace process "would be incomparably worse off without this leadership."
He called for the efforts made by Sinn Fein and the loyalist parties to be recognised, as "they do not always have the freedom of manoeuvre that we imagine".
In his statement Mr Adams said he was conscious of the difficulties faced by unionists. "It had long been my view that republicans need to address the concerns of unionists in a spirit of respect and good will."
However, he described the UUP demand of "no guns, no government" as "not do-able" because it fell outside the terms of the agreement.
"I myself believe that the gap between the UUP demand for decommissioning and what is achievable on this issue appears to be too wide to be bridged in the context of the non-implementation of the agreement."
He said Sinn Fein would not be "dogmatic" on the issue if the Ulster Unionist position could be met. A Sinn Fein source said there was a real fear that if the sides did not bridge the gap over the weapons issue this time, "it may forever block progress". He said it was his understanding that the talks would conclude this weekend if substantive progress had not been achieved.
Commenting on the statement, a member of the UUP review delegation said his party would be making a serious attempt to bring the negotiations to a successful conclusion.
"As the Mitchell review reaches its climax the UUP will concentrate on helping those parties who have yet to reach democracy to cross that bridge," he said.
Mr Sean Farren of the SDLP welcomed Mr Adams's statement and said it was clearly couched in language which expressed the desire to reach an accommodation.
"By acknowledging unionist fears, concerns and pain, the statement goes further than previous Sinn Fein statements," he said.
An Ulster Unionist source said Sinn Fein, by stating that the UUP position was "not do-able", was simply adopting a tough negotiating position and attempting to up the ante in advance of the intense negotiations expected over the next two weeks.
"We thought things were hopeful last Wednesday of getting a commitment on decommissioning. We'll have to wait and see what they say in private to know if this is their answer. But we're still hopeful."
The leader of the Alliance Party, Mr Sean Neeson, said he believed the outcome of the review could still be a success "if the pro-agreement parties work collectively to give the guarantees that seem to be necessary for the impasse to be overcome".