The Taoiseach Bertie Ahern has become the first Irish leader to address a joint session of the two chambers of the British parliament.
Mr Ahern reflected on the Northern Ireland political process and the change in relations between the Republic of Ireland and the UK.
All members of parliament - both the Lords and the Commons - were invited.
He told them: "We must sustain our hard-won achievements on Northern Ireland."
Mr Ahern added: "Remembering where we have come from, we must never, ever, take for granted the stability and the hope that are now taking root in Northern Ireland.
"We have built a remarkable foundation for a whole new level of co-operation between our two countries.
"For decades our relations have been filtered through the prism of conflict. Now, building on the peace and progress of the last decade, we can begin to pay greater attention to the wider partnership of common interests between our two islands."
Mr Ahern praised Tony Blair for his contribution to the peace process in Northern Ireland.
"Tony Blair has been a true friend to me, a true friend of Ireland," he said.
"He has an honoured place in Irish hearts and in Irish history.
"And we thank Prime Minister Blair for ten years of enormous effort to help us get to where we are today. And the fact is we would not have achieved it without his efforts."
Senior figures from the Irish government and Irish opposition leaders were present at the event, as were prominent Irish community members in Britain.
Mr Blair also gave a speech, as did the speaker of the House of Commons, and the speaker of the House of Lords, who were jointly hosting the event.
Mr Ahern was following in the footsteps of Nelson Mandela, Francois Mitterand, the Dalai Lama and Bill Clinton - some of the famous names who have made addresses to the joint Houses of Parliament in the past.