Some time yesterday evening, Sinn Fein and Ulster Unionist Party delegates sat in the same room in Hillsborough Castle and, for the first time since the suspension of the political institutions on February 11th, discussed the issues that divided them in detail.
Paramilitary weapons and police reform were central. It was confirmed quickly that the decommissioning bogey which has haunted the peace process for almost six years might finally be resolved should the IRA issue a statement saying they will put their arms "completely and verifiably beyond use".
The precise wording of the IRA statement, due this weekend is eagerly awaited. It is believed the UUP has been told what to expect: if this is delivered, the decommissioning issue may finally be over.
But all day yesterday, it was the issue of police reform which was uppermost in the minds of the UUP and its leader, Mr David Trimble. Throughout last week the two governments and other parties slowly realised that the UUP was very serious about its demand for the watering down of the decision to implement the Patten report on policing.
The UUP ensured this issue was central to yesterday's talks for two reasons. First, the issue itself is important to it. "Northern Ireland is still British," Mr Trimble said yesterday referring to the Patten recommendation to dilute the "Britishness" of the RUC badge.
The second reason for UUP determination to win concessions over Patten is a simple political calculation. If the party is to agree to go into restored political institutions in exchange for IRA words on decommissioning rather than any arms up front, it needs to be seen to have won some other real concession. The Ulster Unionist Council voted in February to make participation in the institutions conditional on concessions on Patten. Being seen to deliver on this would help Mr Trimble sell a return to the power-sharing institutions to his party.
Today Mr Ahern and Mr Blair are expected to publish a joint letter to the leaders of the pro-agreement parties, outlining in detail how the remaining aspects of the Belfast Agreement are to be implemented, and giving timescales for each item. This will include the all-important issue of the Patten report, and it is here any concessions won by the UUP on the matter will be apparent.
Significantly, Mr Ken Maginnis and Mr Reg Empey left Hillsborough for a period yesterday evening to brief UUP Assembly members on the progress of the talks. In the Sinn Fein delegation at Hillsborough yesterday were a couple of people who are generally seen as very close to the IRA.
In other words, elements of both the UUP and the republican movement whose consent to a deal would be required were being kept fully informed. Speculation about significant developments therefore mounted.
All week the Irish and British governments have been talking down the prospects of success. However, behind the closed doors of Downing Street and Hillsborough Castle, the governments showed considerable urgency about their desire for a breakthrough that would allow for the restoration of the suspended political institutions.
With the May 22nd decommissioning deadline approaching, and this year's marching season coming hot on its heels this latest round of talks was seen as the last chance of progress before autumn. The arrival of a new child for Mr Blair on May 24th is not seen as irrelevant to the amount of time left during which the Prime Minister will maintain this level of personal commitment to Northern Ireland.
Sinn Fein spent seven hours at Downing Street on Tuesday, and was involved in further significant contacts on Wednesday and Thursday. From yesterday morning, Government and SDLP sources were suggesting there was now a new indication of flexibility from within the Sinn Fein delegation. A potential formula had been refined since Tuesday at Downing Street, according to this version of events, that Sinn Fein would be willing to bring to the IRA and urge it to accept.
This formula would involve a more detailed version of what the IRA said to Gen de Chastelain on February 11th last, the day the North's political institutions were suspended. In particular it would involve a more specific, less conditional indication of the IRA's intentions with regard to its weapons.
On February 11th, the IRA led the general to conclude that a process of decommissioning could begin in the context of the full implementation of the Belfast Agreement and the removal of the causes of conflict. What is now expected is a clearer, less coded, IRA commitment.
Meanwhile Mr Trimble continue to put the emphasis on what the republican movement would do rather than say. Suggesting he required details as to what the IRA would do with its weapons, he said: "We are waiting to hear words indicating that they will undertake actions to implement their part of the agreement." Last night Mr Trimble was continuing to call on the IRA to make its intentions clear, a call that is expected to receive an answer within 48 hours or less.
By yesterday afternoon sources were saying all the outstanding issues - decommissioning, Patten, demilitarisation of nationalist areas and criminal justice reform - had been exhaustively discussed. The shape of a possible deal had become clear inside Hillsborough Castle.
All that was required was for Sinn Fein and the UUP to sign up for it. Just before midnight, an exhausted looking Mr Blair and Mr Ahern announced the breakthrough.
Now it is up to the IRA. And if it does what is expected, Mr Trimble has a deal to sell, possibly at an Ulster Unionist Council meeting on May 20th, two days before the institutions are due to be restored.