Sinn Fein's position on weapons decommissioning has moved significantly in the last two days during an intensive series of meetings and contacts with the Irish Government delegation at Stormont.
Since Monday morning, Mr Ahern and his officials have had more than a dozen meetings with Sinn Fein representatives in an effort to devise a new formula to be put to the Ulster Unionist Party.
Late last night, that new formula was believed to include a Sinn Fein statement it was confident it would succeed in influencing the IRA to decommission its weapons by May 2000. This is a considerable advance on Sinn Fein's prior position that it would merely use its influence - as required under the Belfast Agreement - to secure decommissioning. This previous formula gave no prediction of success.
Sources at the talks say the Government delegation has concentrated its efforts on securing a shift in the Sinn Fein position. The change has come out of those meetings.
On Monday, there was a series of formal meetings - including a 75-minute one between Sinn Fein and the two prime ministers - as well as many informal contacts. Yesterday's encounters were much less structured.
Government sources say the Sinn Fein delegation members have displayed a confidence about what they are doing. "They know what they want and where they are going. They are committed to trying to resolve this and they know what their bottom line is," one source said.
A source in one of the political parties not directly involved in the current dispute says there is a clear perception that Sinn Fein has moved, and may be willing to move further. "They have been in and out of the Irish rooms constantly," says this source. "They have been doing real business."
A number of imaginative proposals on how to dispose of paramilitary weapons have also emerged during informal conversations in Castle Buildings. One such proposal - which also surfaced during the failed talks at Hillsborough that ended last April 1st - is that concrete be poured on to the surface of weapons dumps in a manner approved by the head of the International Decommissioning Body, Gen John de Chastelain.
This would have the attraction of clearly putting weapons "beyond use" - as prescribed by the Hillsborough declaration - while not requiring the IRA to "surrender" weapons to the authorities.
While the Irish Government has concentrated on seeking a change in the Sinn Fein position, both it and the British government have also been involved in a series of talks with the UUP. However, sources in both governments and several of the parties say it has not become clear what movement the Ulster Unionist Party is willing to make, and that the UUP engagement with the governments has not been as focused as that of Sinn Fein with the Irish Government.