Northern Ireland's largest Protestant party gave the green light today to the formation of a new Protestant-Catholic government for the embattled British province, a move that means the long-delayed Good Friday peace plan can be implemented in the coming week.

The executive council of the Ulster Unionist Party voted 480 to 349 to endorse a gamble set forth by their leader, David Trimble. Trimble proposed that the pro-British unionists join pro-Irish republicans in the new local government before the Irish Republican Army gives up any of its weapons.

The vote means the Ulster Unionists will drop their boycott of the Northern Ireland assembly and on Monday nominate candidates for the province's 12-member cabinet, which will include two members of Sinn Fein, the IRA's political wing. By the end of the week, the British government will transfer power to the cabinet, and the local government should be up and running.

Today's vote was testament to the negotiating prowess of George Mitchell, the tireless American problem-solver who brokered the Good Friday deal in 1998 and then watched in dismay as the agreement turned to 18 months of angry stalemate.

Mitchell, a former senator, came back here this fall and worked out the blueprint that the unionists endorsed today: immediate formation of the power-sharing local government, followed "soon thereafter" by some sign that the IRA will disarm.

Trimble, who shared the Nobel Peace Prize after the signing of the Good Friday accord, told his party that, by acting first, the unionist side can "place the republicans on the back foot." That's a cricket term, meaning to put the adversary in a tough spot.

Trimble argues that the IRA now must agree to disarm, or it will face opprobrium both in the Republic of Ireland, where Sinn Fein has political ambitions, and in the United States, the chief source of funds for both the IRA and Sinn Fein.

Trimble is a stern, precise former law professor who has not always been a polished political performer. But this weekend, with steady advice from Mitchell and British Prime Minister Tony Blair, he pulled off a clear victory on an issue that had been considered a cliffhanger. His planning even extended to the venue of today's meeting.

Eschewing the faded glory of his party's regular meeting hall in a run-down Belfast neighborhood, Trimble gathered the delegates at Waterfront Hall, a gleaming new glass-and-steel complex rising above the slate gray waters of the River Lagan. From the hall's broad picture windows, you can see the huge yellow cranes of the Belfast shipyard that built the Titanic.

Despite today's decisive vote, everyone here knows the agreement could sink. Trimble promised his party members today that he will resign his post as the province's first minister--that is, its governor--if the IRA has not begun disarmament by February. That would bring the new government to a halt.

Addressing Sinn Fein leader Gerry Adams, Trimble laid out the pending deal in simple terms: "We've done our bit. Mr. Adams, it's over to you. We've jumped. You follow."

Adams said late today that he is unhappy with Trimble's February deadline, but he indicated Sinn Fein is ready to move ahead with the peace process.

The IRA has not yet agreed to give up a single item in its clandestine arsenal, said to include rifles, grenades, bombs, mortars and missiles. But the paramilitary group did announce this month, for the first time, that it supports the Good Friday agreement and is willing to meet with the "decommissioning body," the board overseeing disarmament.

Blair hailed today's vote.

"It's taken a great deal of courage and leadership to get here, and it will take further courage and leadership to build that lasting peace, but I remain of the view that we have the best prospect in a generation for doing so," he said.

In Washington, President Clinton also welcomed the breakthrough. "Beginning next week, government in Northern Ireland is being put back directly in the hands of all the people," Clinton said.

Northern Ireland consists of six majority-Protestant counties that stayed with Britain when the rest of Ireland won independence in 1922. It is a misty land of lush green valleys surrounded by rolling hills. The 1.6 million residents tend to be so outgoing, warm and witty that it's hard to believe they have been killing each other for three decades over a political dispute.

The province is split between unionists, who want to remain part of Britain, and republicans (also called nationalists) who want to form a single nation with the Republic of Ireland.

The basic argument is about territory and power, not religion. But because most unionists are Protestant, and most republicans Roman Catholic, ancient sectarian animosities have amplified the differences.

For 50 years, the Protestant majority in the northern counties maintained a system of preference and discrimination not unlike the Jim Crow societies of the American South. Finally, in 1972, the British government shut down the Protestant-controlled local parliament and set up direct rule by the national government in London.

One of the strongest attractions of the Good Friday agreement has been its promise of "devolution"--the transfer of local governing power to a new Northern Ireland assembly, which is to include members of both religious communities. That would give Catholics a say in province-wide government for the first time.

But the rules of the assembly require that both communities approve any major action. That meant Trimble's Ulster Unionists have been able to block implementation of the plan simply by refusing to participate.

When the new assembly gathered for what was supposed to be its historic first meeting in July, Trimble's party boycotted the session. Angry and frustrated, the remaining parties fell into a bitter exchange of taunts and insults.

The next session of the assembly is scheduled for Monday, and the atmospherics should be different this time. By agreeing to participate, Trimble's party will make the new government a reality. At Monday's session, the four largest parties--two unionist, two nationalist--will nominate members to the provincial cabinet.

Meanwhile, the IRA will be expected to name a representative to meet with the disarmament panel. That should happen by the end of the week.

On Tuesday, the focus will shift to London, where Parliament is to enact legislation transferring local governing power to the assembly in Belfast. After that--perhaps on Thursday--the Northern Ireland assembly will start its official work.

After a year and a half of standing still, Northern Ireland seems ready to race ahead to its new era of self-government, power-sharing and disarmament. But given the tensions here, the route probably won't be trouble-free.

"No one should think," George Mitchell said tonight, "that this process will now all be sweetness and light."