The Taoiseach hopes his promise to "crush" republican dissidents who would try to undermine the Belfast Agreement will encourage unionists to vote Yes in Friday's referendum.
Mr Ahern has also defended the decision to allow the Balcombe Street IRA prisoners attend the Sinn Fein Ardfheis because "they did convince a substantial number of people away from violence".
Mr Ahern was speaking after being awarded an honorary doctorate in laws by Boston College in recognition of his efforts to secure peace in Ireland. Cardinal Bernard Law of Boston told the more than 3,000 students graduating yesterday to see the Taoiseach as an inspiration in peacemaking.
Asked later how he might win greater unionist support for the Belfast Agreement, the Taoiseach said he had now "spelled out our view of what will happen if this agreement is in place and if republican dissidents continue to try and knock it off course or undermine it in any way."
He referred to the statement he had released earlier in which he said that if the people of Northern Ireland and the whole island "vote emphatically for the Good Friday agreement, this will represent an unassailable democratic verdict from any point of view".
This would "quite literally cut any remaining ground from any organisation that would in defiance of all the people for any reason persist in violence. If there is any such violence, it will be firmly crushed."
The Taoiseach said: "It is important to the unionist people to know that we are not prepared to allow this violence by dissident groups to drift on. We have always tried to deal with this over the years but we are in a different situation now. The main groups of paramilitary loyalists and Sinn Fein will not be engaged in violence and that is certainly our understanding from what they said and their commitment to the agreement.
"Therefore we are dealing with more isolated groups and it is easier for us . . . to crush those people who will endeavour to upset the agreement."
Asked about the negative effect on unionist opinion caused by Sinn Fein's welcome for the Balcombe Street prisoners, the Taoiseach was unrepentant about the decision to release them for 24 hours. But he regretted the "triumphalist" reception. He had understood they would not be allowed on the platform and he believed the Sinn Fein leadership did not think so either.
But he said they had convinced "a substantial number of people to move away from violence. So the question has to be asked: do four people getting out for 24 hours that convince several hundreds of young people away from violence, is that better than keeping them in for 24 hours and allowing those hundreds of young people continue on their road of violence?"
Mr Ahern answered his own question: "I would contend we have to be sensible and mature and, as difficult as it is, we have to try and convince people. I can't convince some of these people. But if the Balcombe Street four can say that the way they followed 24 years ago was wrong and that young people should disengage from that kind of activity, then I think there is a positive side."
Asked how damaging a unionist No vote would be, Mr Ahern said: "It would make life difficult as we move into the next phase of the assembly."