The Civil Public and Services Union has voted to campaign against the fiscal treaty in the upcoming referendum.
The vote, which was almost unanimous, was taken at the union's annual delegate conference in Cork.
Earlier, Tánaiste Eamon Gilmore rejected claims that alternative funding would be available to Ireland if the country rejects the fiscal treaty.
Speaking on RTÉ's Morning Ireland, Mr Gilmore reacted to comments made earlier by NUI Galway economics professor Terrence McDonough.
"He talked about, for example, that we would have access to the EFSF.
"The EFSF is going to be overtaken by the ESM and it won't be possible for new applications to be made to the EFSF once the ESM comes into operation.
"And he seemed to be suggesting that somehow, somewhere, somebody would come to our assistance."
Professor McDonough said he believes alternative funding would be available to Ireland if the country rejects the treaty.
He said: "Europe is not actually going to ultimately deny Ireland funding. Ireland is small, but scary. Europe did not allow us to let a bank go down.
"I think there will be funds available outside of the ESM."
Prof McDonough also said that the treaty was being proposed as a "substitute" to changes that actually needed to be made, such as to ECB reform, debt relief and euro bonds.
He said that the adoption of the treaty and the budgetary rules within it would not make these kinds of changes unnecessary.
Sinn Féin Deputy leader Mary Lou McDonald has claimed that funding will continue to be available to this State should we reject the proposed treaty.
Ms McDonald said that all the evidence was that Europe and the EU had strived "might and main" not to allow any country go under.
If Ireland succumbed to its debt burden, she said it would mean very big trouble for the euro.
But Fianna Fáil leader Micheál Martin has insisted that access to the European Stability Mechanism was critical to Ireland's recovery.
He said that relying on any alternative source of funding would prove more expensive and increase the chance of this country requiring another bailout.
The fiscal treaty referendum will take place on Thursday 31 May.
Merkel rules out renegotiation of treaty
German Chancellor Angela Merkel has ruled out any renegotiation of the treaty and rejected criticism that the European Union was not focusing on growth.
"Parliaments all over Europe are about to adopt it. Ireland has a referendum on it at the end of May. It is not renegotiable.
"The fiscal pact has been negotiated. It was signed by 25 government heads and is already ratified by Portugal and Greece."