The Taoiseach, Mr Ahern, issued a warning last night about attacks on the Northern Ireland peace process by breakaway republican groups.
He referred to such organisations as "pathetic prisoners of the past" and hinted that tough laws introduced in response to the 1998 Real IRA bomb attack on Omagh, Co Tyrone, would be used to crush any renewed terrorism.
Speaking after the rail line bombing in Co Armagh, Mr Ahern said he had no quarrel with "democatically-expressed and argued opposition" to the Good Friday agreement.
But he added: "Those involved in dissident paramilitary organisations should recognise now, before they commit themselves any further, the hopelessness and futility of trying to defy the will of the people from whom they have no mandate."
He added: "The full weight of the law will be applied to any revived activity that could put lives at risk. And we will not hesitate to make swift use of legislation passed after the Omagh bomb - if that should prove necessary.
"Unfortunately there are a handful of sad and pathetic men and women who are prisoners of the past, when everyone else has moved on, and who cling to ideological simplicities.
"Irish unity cannot be achieved by coercion," he stressed. The Taoiseach was speaking in Gorey, Co Wexford, at a lecture to honour the late Gordon Wilson, whose daughter Marie died in the 1987 Enniskillen bombing, and who afterwards became a member of the Seanad.