SIPTU's 63,000 public service members will begin voting on the Croke Park II deal next week after their union leaders urged them to back the agreement.
In a major boost to the Government, the country's largest union recommended a yes vote to proposals to cut €1bn from the state payroll over three years.
The Prison Officers Association, which had been opposing the deal, has also told its 3,300 members to give it the thumbs up.
It rejected the deal as a member of the 24/7 Frontline Services Alliance, but changed its stance after striking a standalone pact with the Government protecting its members' premium pay and allowances.
Following two days of debate, SIPTU said it decided members were better off in a national agreement that prevented compulsory redundancies, and curbed redeployment and outsourcing.
General President Jack O'Connor said the union was firmly recommending the proposals, but it was not telling public servants to vote yes because the deal was good for them.
"We are asking them to vote yes to keep the protections of the Croke Park agreement in place," he said.
In a statement, the union's National Executive Council said it firmly believed "the best way public service workers can protect their interests is through a single centralised agreement".
"Otherwise each group, grade and category, irrespective of how strong they believe themselves to be, is vulnerable to being isolated and attacked in the desperate quest for savings," it said.
The senior officials said the proposals for a revised agreement on pay was the result of "unprecedentedly difficult negotiations" following "the most serious economic collapse in the history of the State".
"Voting no will not make the problem go away," they said. They said the Croke Park Agreement would collapse, and members would still be faced with the cuts without its protections.
However, they accepted that a protracted industrial campaign would present a major difficulty for the Government and would become a "gigantic trade dispute".
SIPTU said it aimed to "recover lost ground later" and assured members it would not participate in any renegotiation of the deal that meant more cuts.
The union's verdict is crucial to the future of the cost-cutting proposals that have been shot down by large groups of staff, including nurses, gardai and paramedics.
It enjoys the largest share – or 25pc – of the public sector union vote.
IMPACT, which holds another 21pc and the PSEU, with a 4pc share, have also recommended a yes vote.
This means unions holding more than a 50pc share of the vote are now recommending a 'yes' vote.
However, the final decision rests with the country's 290,000 public servants.
They will decide in the coming weeks whether to back their leaders' views. Their final verdict on the deal – which will decide if it is ratified – will be revealed on April 17.
The Association of Higher Civil and Public Servants (AHCPS), the Association of Secondary Teachers of Ireland (ASTI) and the Civil, Public and Services Union (CPSU) are opposing the deal.
UNITE, the Irish Federation of University Teachers (IFUT), Irish Medical Organisation (IMO), Irish Nurses and Midwives Organisation (INMO), and Teachers Union of Ireland (TUI) have also advised members to vote no.
Meanwhile, four unions unveiled a national campaign to derail the deal and force the Government back to the negotiating table.
Over 1,000 workers vowed to veto the deal at a mass rally organised by the INMO, IMO, CPSU, and UNITE.
Last night's rally at Cork City Hall is the first salvo in a nationwide campaign to have the deal rejected and renegotiated.