Members of the Manufacturing Science and Finance union have voted by almost two to one to accept the Programme for Prosperity and Fairness. The fourth-largest union in the Republic, MSF represents technical, craft, professional and managerial grades across a wide sector of the economy.
The MSF result is the first serious indication of how trade unionists are expected to vote on the new agreement.
Despite the high profile of the anti-agreement campaign, it confirms the general feedback within trade unions that a majority of delegates to the special delegate conference of the Irish Congress of Trade Unions next Thursday will be mandated to accept the PPF.
The final outcome should be clear by Monday, when results for several of the State's largest unions, including SIPTU, IMPACT and the Irish Nurses' Organisation, will be announced.
Support for the agreement within MSF was highest in the private sector, where 75 per cent of members are based. The private sector voted 70 per cent in favour of the PPF, compared with under 60 per cent in the public sector.
Overall, 65 per cent of members supported it.
MSF's Irish secretary, Mr Jerry Shanahan, said the vote should not be seen as a "carte blanche" for the PPF. "In particular, MSF members are worried about the rising rate of inflation. In this context, the fact that the agreement provides a review mechanism was an important consideration.
"Affordable housing and a quality public transport service were also issues constantly raised throughout the balloting process. It is imperative that substantial progress be made on these matters over the next three years."