The people of Northern Ireland have, since the 1998 peace agreement, so profoundly changed the province and defied its quarrelsome politics that the Irish Republican Army's action renouncing violence became inevitable, former Sen. George J. Mitchell, the broker of the accord, said yesterday.

"They have seen the benefits of peace and normality," he said. "They couldn't go back to the way it was, and, in my judgment, no politician could ever take them back."

"In effect, the public has dragged the politicians along in their wake."

The proposed new government in Belfast, designed on a formula that divided power between the Protestant majority and the Catholic minority, was the centerpiece of the agreement, but it had only a short-lived existence in 1999. Repeated efforts to get it up and running since then have repeatedly fallen victim to political feuding.

But during that time Northern Ireland's society surged forward, capitalizing on the agreement and the end to systematic sectarian violence that resulted from cease-fires by the I.R.A. and its Protestant paramilitary counterparts.

"A lot of people now have a lot invested in Northern Ireland not going back to what it was," said Richard English, professor of politics at Queen's University in Belfast.

"There was critical mass pressure on politicians," he added, "saying whatever you do, we don't want to go back to the time when you saw a hotel being built and assumed it would be blown up a month later and every time you switched on the radio somebody had been murdered in the city center."

Downtown Belfast used to be a dark place of derelict buildings and shuttered storefronts where venturing out in the soccer jersey of a team representing one "tradition" in Northern Ireland could cost the wearer's life from a gunman of the other "tradition."

These days, construction cranes dot the horizon, stately old courthouses have been restored, riverside hotels and convention centers have risen and the downtown streets are thronged with people who have returned to the province after departing in despair over past decades.

"It used to feel as if there were a curfew after dark," said Mr. English. "Now people fly in from England and Scotland for the weekend night life."

Mr. Mitchell, in a telephone interview from Maine, said he had been in Belfast two weeks ago and found it "completely different."

"When I first started going there, you never saw a crane, there was little construction anywhere and everything was generally negative," he said. "Now there has been a total economic transformation, with unemployment rates lower than they have ever been and a tremendous energy."

Martin McGuinness, chief negotiator of Sinn Fein, the I.R.A.'s political wing, acknowledged the importance of the changes on the ground though insisting that the credit for producing them belonged to the party's leader, Gerry Adams.

Speaking from Washington, where he was briefing members of Congress, Mr. McGuinness said, "There is no doubt that people felt much less afraid than before, much more confident about building their lives and businesses and that, despite the problems besetting the political process, they believed the peace process would continue."

David McKittrick, author of many books on Northern Ireland, said that for the I.R.A., which had always resisted any move that might connote surrender, the appearance of peace on the ground offered a defensible way out. "In giving this up, they weren't beaten, they were given an alternative way to reach their goal," he said. "The guns had become an obstacle and the alternative was politics, not capitulation."

Throughout the day yesterday, residents and politicians, long familiar with dashed hopes, spoke about the I.R.A. move with a wariness that even emerges in the harsh accent of Northern Ireland, a crabbed and unforgiving version of the lilting way people speak south of the border.

"Everyone is in some way haunted by the Troubles and the sense that things might end up badly, and they don't allow euphoria to get the better of them," Mr. English said. "Northern Ireland is a place where people always suspect the worst motives; they don't go leaping about in the streets."

Mr. English, the author of an authoritative 2003 history of the I.R.A., said that its statement was "as clear a declaration of the end of the war as you're going to get."

But he warned that progress would still be slow. "Given the problems of trust and timing in Northern Ireland, we will need at least five years to see whether this is the beginning of moving to complete peace," he said.

He also cautioned that peace in Northern Ireland came with limitations. "This remains a sectarian place where differences are dealt with somberly and people don't necessarily like each other," he said. "But they no longer go around shooting people - they believe differences can be dealt with in a nonviolent way."

He said that people used to fear coming to Belfast from London, but now the reverse was true. "Belfast," he said, "has ceased being famous for what it was famous for."