Some 86 per cent of Green Party members tonight voted in favour of entering a Fianna Fáil-led coalition. It will be the first time the party, founded 25 years ago, will have been in government.
Around 350 Green Party members meet this afternoon in Dublin to discuss the coalition proposal, ahead of tonight's vote.
More than 500 members were at the Mansion House in Dublin since early afternoon to be briefed on, and debate, the terms of a draft programme for government hammered over 10 days of negotiating.
The party leadership was in buoyant mood after the overwhelming endorsement - only 67 members voted against the proposal, with 441 voting in support.
But before speculation could shift to the position of the party leader, Trevor Sargent announced his resignation from the position.
While he never ruled out joining a Fianna Fáil coalition during the election campaign, Mr Sargent had said he would step down if it happened.
Speaking after the votes were counted, Mr Sargent said: "This, my friends, is the proudest day of my life. It's the day when courage won out.
"It's a day when a hunger for change won out over the status quo."
Having been criticised for considering joining forces with Fianna Fáil, notable by the party's founder and first TD, Roger Garland, Mr Sargent struck a conciliatory note.
"Those who voted no, also made a courageous decision," he said.
"I'm stepping down on the basis that I said I would step down. My word is my bond. I am honour-bound to step down.
"I brought this party to the crossroads and I think it is now time..."
The process to elect a new leader would be a "step-by-step" process starting with a meeting of its executive committee, Mr Sargent said. While he did not rule out a return to leadership, he stressed that "being a leader took time" and "there's plenty of talent there to have a new leader."
Among the commitments the Greens are signing up to is the introduction of new targets to cut emissions, a carbon tax, increased investment in renewable energy, an analysis of the transport sector based on environmental concerns and increased spending on education.
Green Party negotiator and TD John Gormley thanked the party delegates for their " overwhelming support". Making the decision had encompassed a range of emotions, from "joy to tears", he said.
But he said going into coalition with Fianna Fáil was the only way Green Party policy could be implemented. "I hope that we can full the expectation, a heavy burden falls on our shoulders," he said.
Green Party negotiator Dan Boyle admitted this afternoon that some members had concerns about the proposal. "People are expressing areas they have disappointment..." he said.
The "good atmosphere" he spoke of was not spoiled by anti-war demonstrations outside the Mansion House where protestors pointed out the party's glaring u-turn by not obtaining an end to the continued use of Shannon airport by Iraq-bound United States military flights.
Former MEP Patricia McKenna was one of the most high-profile of opponents to the coalition deal on grounds of the party's vehement stance on the issue.
The Greens have also accepted that the controversial M3 motorway which is to run near the Hill of Tara, will go ahead, despite its previous vociferous opposition to the plan.
Fianna Fáil refused to offer any concessions to the Greens on another one of its key election platforms - the demand for an end to political parties and politicians accepting corporate donations.
This morning, founder of the Green Party in Ireland Roger Garland warned that a deal with Fianna Fáil could lead to the "wipe-out" of the party. Mr Garland said he felt "betrayed" by the current leadership and described concessions made by its negotiating team as "unbelievably bad".
But the European Green Party today urged Irish members to vote in favour of entering government.
Mr Sargent and the party's other TDs - John Gormley, Mary White, Eamon Ryan, Paul Gogarty and Ciaran Cuffe - will vote in favour of Bertie Ahern as Taoiseach when the 30th Daíl reconvenes tomorrow.