The Communications Workers' Union has voted in favour of supporting the fiscal treaty in the forthcoming referendum.
Almost 60% (4,826) of members voted Yes, with 3318 against.
In his address to delegates in Galway, ICTU General Secretary David Begg said Europe is in deep trouble and needs massive growth stimulus to deal with public and private debt.
He described the fiscal treaty as a poison pill, saying failure to ratify would mean no access to ESM funds.
The issues are more complex than slogans on current campaigns on the treaty suggested, he said.
Mr Begg also called on delegates to back the new French president, who he said embraces fiscal consolidation in its proper context.
Austerity has become a mantra without meaning and he said Ireland should assist Francois Hollande to achieve his stated objective of a growth strategy.
Mr Begg said the referendum merely allows the Government to sign up to the treaty.
If the referendum cannot be deferred, he said people should be given assurances that the treaty will not be ratified until the end of the year.
Earlier today, the Minister for Public Expenditure and Reform said the campaign for a Yes vote was going very well.
Brendan Howlin was speaking before the start of the Association of County and City Councils' annual conference in Kilkenny.
He said a Yes vote was important to the future stability of their money.
Meanwhile, the Minister for Jobs, Enterprise and Innovation said a Yes vote will keep Ireland on the map for foreign direct investment.
Richard Bruton said 2011 had been the best year in a decade for investment in Ireland by foreign companies.
He said being at the heart of Europe is crucial to Ireland's job opportunities.
Referendum booklet misleading: Sinn Féin
Sinn Féin has accused the government of deliberately misleading the public with their referendum booklet that is being distributed throughout the country.
TD Caoimhghín Ó Caoláin said Sinn Féin believes the booklet is designed to confuse and mislead as the Government does not spell out the consequences for Ireland's economic sovereignty in terms of enforcement and punishment under Articles 5 and 8 of the treaty.
Mr Ó Caoláin said Article 5 provides for the European Commission to take a hands on role in terms of budgetary decisions here and Article 8 provides for the European Court of Justice to punish the state for failing to adhere to the treaty terms.
He also said producing such a booklet is a waste of taxpayers' money and reiterated the government has not explained the consequences of adopting the treaty on 31 May.