The Taoiseach has said the attribution of last month's robbery at the Northern Bank headquarters in Belfast to the IRA represents a serious setback for the political process in Northern Ireland.
Earlier, the North's Chief Constable, Hugh Orde, said he believed the group was responsible for the raid, and also revealed that a total of £26.5m (€37.5m) was taken and not the £22m previously reported.
The Taoiseach said the development underscored the need for compelling commitments, in word and deed, that the full spectrum of IRA paramilitary activities and capability has been brought to a definitive closure.
Sinn Féin's chief negotiator, Martin McGuinness, said the IRA had told him they had not taken part in the raid.
He said the allegations were part of an attempt to undermine the peace process and he was confident no member of the IRA was involved.
Following Mr Orde's news conference, the Northern Bank confirmed that most of its existing notes, totalling £300m, would be replaced with new ones. The bank's polymer £5 notes will not be affected.
Mr Orde spoke publicly about the raid for the first time after coming under pressure to state if he believed there was a definite republican connection to the robbery.
Earlier, the Chief Constable briefed senior members of the policing board - its Chairman Sir Desmond Rea and his deputy Denis Bradley - on the investigation into the raid.
Mr Ahern said he was concerned that the robbery could have been planned while he was in negotiations with people who would know the leadership of the Provisional IRA.
He added that there had been setbacks before, but that he would not stop his efforts now.
Downing Street said the British Prime Minister, Tony Blair, took the development very seriously.
Two weeks ago, an anonymous republican source denied any IRA link and Sinn Féin's Gerry Adams said he believed that to be true.
A month ago, Mr Orde met Sinn Féin leaders for the first time, when an historic political breakthrough seemed possible.