George Mitchell and Bertie Ahern are among the guests
Some of the main players who helped negotiate the Good Friday Agreement are in Belfast for a conference to mark its 10th anniversary.
But the significance of the event has been reduced because of the absence of others, including ex-president Bill Clinton and former PM Tony Blair.
The guest list includes Bertie Ahern, on what will probably be his last visit to Northern Ireland as taoiseach.
Ex-US Senator George Mitchell and General John de Chastelain are there.
The conference will mark the progress of the peace process in the past decade.
Speaking ahead of the event, Mr Ahern said: "It's a happy day in that what we have achieved over 10 years ago, and really the work that went into that final day has proved to be enormously successful.
"I think Northern Ireland can never look back again."
Tony Blair and Bill Clinton are not attending
Mr Mitchell said: "It's helpful to review the past and to mark progress where it has occurred as it truely has in Northern Ireland.
"The most important focus should be on the future."
Organisers wanted as many as possible of the leading figures who played a part in the Agreement to attend.
Mr Clinton said the Agreement paved the way for political stability.
"I think that everybody who was part of the Agreement knows that they did it," he said.
"They know in their bones that they struck a blow for a better future for their kids."
It is understood he is concentrating on his wife Hillary's White House campaign, while former prime minister Tony Blair has a clash of dates involving a family birthday.
Mr Blair said the deal had inspired people across the world.
"They see it as a real beacon of hope for other such conflicts they think it is amazing that it's happened," he said.
"I think it is a really great symbol of how the world changes so fast and can throw up the opportunity to settle conflicts that seemed irresolvable for decades, centuries even."
Mr Blair will be in Dublin on Friday for a dinner where he is being honoured for his part in the peace process.
Deputy First Minister Martin McGuinness has also pulled out because he is visiting the United States, as is First Minister Ian Paisley, whose party is boycotting the event.