LONDON - Irish Prime Minister Bertie Ahern yesterday became the first leader from Ireland to address the joint houses of the British Parliament, praising Britain and its departing leader, Tony Blair, for promoting peace in Northern Ireland.

He warned, however, that the week-old coalition of British Protestants and Irish Catholics in Belfast represented a fragile start to healing communal divisions after four decades of bloodshed that claimed 3,700 lives.

"We must sustain our hard-won achievements on Northern Ireland," Ahern said. "Remembering where we have come from, we must never, ever take for granted the stability and the hope that is taking root."

Ahern became the 31st foreign leader to get the opportunity to address the combined houses since the tradition began in 1939. He received a standing ovation recognizing his critical role in the peace process.

In a speech that covered eight centuries of Anglo-Irish conflict to this month's power-sharing breakthrough in Belfast, Ahern said the last decade of peacemaking overseen by himself and Blair had shown the world "that even the seemingly intractable can be overcome."

Blair said he had met many great world figures, "but I've never met a bigger one than Bertie Ahern."

Blair and Ahern both gained power in 1997 and worked together in a search for peace in Northern Ireland, a part of Blair's United Kingdom. They signed a landmark 1998 peace pact in Belfast and oversaw several summits with Northern Ireland leaders from 1998 to 2006. And they were on hand last week in Belfast when a Catholic-Protestant administration was formed.

Blair, 54, has announced he will step down June 27, while Ahern, 57, and his Fianna Fail party face a struggle to retain power in a May 24 election.

Ahern's audience included about 400 lawmakers from both houses, including seven former British secretaries of state for Northern Ireland stretching back to 1972.