The Irish Republican Army pledged today to open negotiations with a disarmament commission, taking a key first step toward eventually surrendering its weapons in support of Northern Ireland's peace accord.

The IRA promised in a statement to send a negotiator to the commission if the British province's major Protestant party, the Ulster Unionists, first formed the Protestant-Catholic administration envisaged in last year's Good Friday agreement.

The policy turnaround came after Ulster Unionist leader David Trimble said in a crucial concession Tuesday that such an IRA statement would be sufficient for him to accept the Sinn Fein party, the IRA's political arm, within a new cabinet. Trimble's resistence to doing that based merely on a pledge by the IRA had long been the stumbling block to progress.

These back-to-back compromises were carefully prescribed as part of American mediator George Mitchell's 11-week mission to save the accord.

Britain's government minister for Northern Ireland, Peter Mandelson, called the IRA pledge "welcome, and stronger in certain respects than some expected."

In Dublin, Irish Prime Minister Bertie Ahern said the IRA's words "carry very real value at this critical time."

However, the IRA statement offered no explicit guarantee that gradual disarmament will follow. But until now the outlawed guerrilla movement had rejected any direct contact with the disarmament commission, formed in 1997 during peace negotiations. IRA commanders had argued that to hand over even a single bullet would symbolize surrender, and humiliate and split their ranks.

Sinn Fein leader Gerry Adams praised the IRA for demonstrating "courage, discipline and patience." He said "no one should underestimate the effort that this initiative involved."

Adams's diehard predecessor as Sinn Fein leader, Ruairi O Bradaigh, predicted a new generation would take up arms in pursuit of the traditional IRA goal--the abolition of Northern Ireland as a Protestant-majority state linked with Britain.

Trimble faces a far more serious challenge in the immediate future. His decision to soften his "no IRA guns, no government" policy needs approval by a majority of the Ulster Unionists' 800-member governing council. The vote is expected Nov. 27.

If Trimble wins, his lawmakers could then elect Sinn Fein politicians to two of the intended cabinet's 12 posts in early December.

The most unpredictable phase would come in January when John de Chastelain, the Canadian general who leads the disarmament commission, is expected to announce whether the IRA has begun identifying the whereabouts of its many weapons depots.

According to sources within the negotiations, who spoke on condition of anonymity, the IRA representative to the commission will be Brian Keenan, the group's reputed senior commander. Police say Keenan, 57, was instrumental in opening the IRA's most important arms-supply link in the early 1970s with Libya. In speeches he has denounced the idea of disarmament.

CAPTION: Sinn Fein leader Gerry Adams, foreground, with party colleague Martin McGuinness, praises move by IRA toward surrendering its weapons.