The Taoiseach, Mr Ahern, and the British Prime Minister Tony Blair have warned the IRA that it must give up all criminal activity if there was to be any return of power sharing to Northern Ireland.
Mr Ahern and Mr Blair pose for photographs before today's meeting in Downing Street.
Following talks in Downing Street the two men said the IRA's continuing criminal and paramilitary activity was the sole remaining obstacle to a peace settlement in the North.
Mr Blair and Mr Ahern were briefed by PSNI Chief Constable Hugh Orde and Garda Commissioner Noel Conroy on the £26.5 million Belfast bank raid.
They said they accepted the claim of both police forces that it was the work of the Provisional IRA.
"The obstacle now to a lasting and durable settlement in Northern Ireland is the continuing paramilitary activity and criminal activity of the IRA," Mr Blair said.
"It has got to stop. It has got to stop in its entirety. There cannot be any compromise with that.
"If it is given up the process can move forward on an inclusive basis."
His words were echoed by Mr Ahern who told reporters in No 10: "The reality of the situation is that until we have got an end to criminality we cannot win the trust and confidence of the collective parties to move forward."
Mr Blair acknowledged that the bank raid may have been the result of divisions within the republican movement. However, he said Sinn Fein and the IRA were now alone in not accepting that there must be a commitment to exclusively peaceful and democratic means if the process was to move forward.
He said the unionist community now accepted the principle of power sharing with nationalists provided they abandoned criminality and paramilitary activities. "There cannot be any going back on that," he said.
Mr Blair said he hoped there would be a period of "hard and difficult reflection" by the republican leadership. "There is almost a simplicity about the present situation," he said.
"There is no way forward by compromise, fudge, ambiguity on this issue any more. "There is only one way forward. Everybody gets on to the democratic bus and goes forward on that track or not."
Mr Ahern added: "The questions are very clear. They are very simple. If they (republicans) are prepared to engage and are prepared to move forward we can get on but we need that response back from them."
Sinn Féin chairman, Mr Mitchel McLaughlin, later dismissed the comments,
saying there was nothing new in them.
The Foyle Assembly member criticised Mr Blair for failing to acknowledge the DUP had walked away from power sharing in the talks last December.
"Mr Blair is well aware of Sinn Féin's commitment to peaceful and democratic
means," he said.
"Sinn Féin has been the engine driving the peace process for the last decade.
"Sinn Féin's commitment is to make this process work. But as the Taoiseach and
the British Prime Minister know we cannot do this on our own."
The SDLP leader, Mr Mark Durkan, who also held talks with Mr Blair today, welcomed the "clarity" of the statements.
"It is not a matter of sending a signal that we are moving against any one party at this stage or that the rest of us are looking to move ahead without any one party at this stage, but making clear that we are not waiting until we can all only move forward only with the say so of the IRA," he said.