President Clinton on Friday expressed optimism about peace in Northern Ireland as controversy over his affair with Monica Lewinsky dogged the southern leg of his whirlwind Irish tour.

Clinton, his spirits lifted by the warm reception he received across Northern Ireland on Thursday, spoke of his "personal passion" for the Irish question and said the world owed Ireland a debt of gratitude for showing the way to peace."I hope you will continue to labor for peace here because if we can complete this peace process you can't imagine what it will enable the United States to do in trying to stand up for peace in other parts of the world where people have fought over their religious, their racial, their ethnic, their tribal differences.

"I can always then say `no, no, no, look at Ireland' when they tell me it can't be done," Clinton told an audience of government officials, businessmen and trade unionists in Dublin.

Later during a flight to Adare in the west of Ireland, Clinton told journalists the peace process was much more advanced than when he last visited Ireland in 1995. But he added: "It is also at a critical juncture."

On Saturday the president planned to round off his three-day Irish visit by making a speech in nearby Limerick before fulfilling a long-held ambition to play golf on the fabled Ballybunion course.

The president made a point of saying how glad he was to be back in Ireland, where he is a popular figure on both sides of the border because of his commitment to the peace process.

But Clinton's good mood was partly soured by withering criticism back home of his extramarital relationship with Lewinsky, a former White House intern. The scandal has shaken his presidency to its foundations and could yet lead to his impeachment.

A day after Connecticut Democratic Sen. Joseph Lieberman said Clinton's behavior had been "immoral," the president tried to draw the sting out of the onslaught by apologizing for the affair for the first time.

"I can't disagree with anyone else who wants to be critical of what I have already acknowledged was indefensible," a subdued Clinton said. "I've already said I made a bad mistake and it was indefensible and I'm sorry."