The Taoiseach has paid tribute to David Trimble who announced his resignation as leader of the Ulster Unionist Party yesterday.
Mr Ahern said Mr Trimble had made a tremendous contribution both during and after the Good Friday Agreement.
The British prime minister Tony Blair said history would show that without David Trimble's period as leader of the UUP, Northern Ireland would not have become the changed place it was.
Mr Trimble announced his decision to quit after a private meeting with his party President and Chairman.
"I indicated to them that I do not wish to continue as leader," he said afterwards in a statement. Chairman James Cooper will set in motion the process of finding a replacement who will be elected at a meeting of the Ulster Unionist Council.
Mr Trimble said he would formally resign at that meeting. He thanked party members for their support over his ten year leadership. "There have been difficult times, but also times when we have been able to make a difference.
"I have no doubt that Northern Ireland is a much better place and Unionism greatly advantaged because of our efforts," he said.
New Northern Ireland Secretary Peter Hain praised Mr Trimble for demonstrating "tremendous commitment and courageous leadership."
"He brought Unionism along a new and uncharted road that led to the signing of the Belfast Agreement and, subsequently, to the initial establishment of devolved Government in Northern Ireland," he said.
The SDLP's Mark Durkan said Mr Trimble deserved credit for concluding the Agreement and "reaching an historic deal with Irish nationalists as to how we can share this island together." But he also said it was a pity that while he fought the battle within unionism for the Agreement "he never went for its wholehearted implementation."
He had been "bamboozled by Downing Street into a a process of side deals that undermined the Agreement and ultimately undermined him," said Mr Durkan.
However Jeffrey Donaldson, the Lagan Valley MP who long battled with Mr Trimble before jumping ship 17 months ago and joining the DUP, said his resignation was inevitable. He said Mr Trimble's position had become untenable and he was not surprised he was going. He said Mr Trimble had failed to recognise the shift of opinion within unionism and the growth in opposition to the Good Friday Agreement.
Additional reporting PA