IRA hardliners refused last December to let Sinn Féin's Mr Gerry Adams and Mr Martin McGuinness sign up to promises not to engage in criminality, the Taoiseach has said.
In an attempt to ease pressures on the Sinn Féin leadership, the Taoiseach said he believed they are "genuinely saying" they want to sign up to a final peace deal.
However, Mr Ahern warned that the Sinn Féin leadership has to deliver major concessions because the marching season "is just weeks away" and it could "unravel" the peace process.
Mr Ahern has taken a deliberately different and softer line to the Minister for Justice, Mr McDowell, who once more alleged Mr Adams, Mr McGuinness and Kerry South TD Mr Martin Ferris are members of the IRA's army council.
"I might have my own views about whether people are, or are not, on the army council. I do not know the membership of the army council," said Mr Ahern.
Offering diplomatic words towards the Sinn Féin leadership, Mr Ahern said: "They are genuinely saying that they are trying to get to a situation where they want to see these things through," said Mr Ahern, speaking in Dublin.
"Why did we break down on the eighth of December on the criminality end? We know about decommissioning. But why did we break down on criminality? It is because Sinn Féin went to the IRA and the IRA would not allow them the movement. That is what happened. Sinn Féin went to the IRA. But Sinn Féin has to work that out," the Taoiseach said.
"If [ the SF leadership] want this process to be inclusive they have to come back to us as we asked some time ago and give us the answer," said Mr Ahern, who acknowledged that Sinn Féin would need time. "It requires some time. I'm not saying that we will leave it forever. We need them to be coming back saying, the end of paramilitarism, the decommissioning of all weapons, and the cessation of all criminal behaviour."
