SIPTU has warned that it will only back the fiscal treaty if the Government agrees to implement a €10bn stimulus programme.
The programme would be based around investment from pension funds to create "tens of thousands of jobs”, according to the country's largest union.
Responding to SIPTU's position, Taoiseach Enda Kenny said it was not about people being induced to vote by issues of money.
Speaking in Belfast, Mr Kenny said the "train for the future of Europe" is leaving on 1 June and he wants the country to be on it.
A SIPTU policy document described the strategy underpinning the treaty as a one-sided austerity approach, principally at the expense of working people, which would not work.
The National Executive Council of SIPTU, which met today, said a more rational route would be to share the burden and foster growth, with a less severe "debt brake".
It called for what it described as an "off-balance sheet domestic stimulus plan", in addition to projected capital spend.
SIPTU says this would be based on elements including investment from private pension funds incentivised through exemptions from the pension levy.
SIPTU President Jack O'Connor said Ireland was “between a rock and a hard place”.
He said the treaty would impose an unnecessarily severe debt brake - but if Ireland did not ratify it, the country would not have access to the ESM bailout fund.
However, Mr O’Connor said that if the Government committed to the stimulus plan, SIPTU would go along with the treaty despite their reservations, because it would give the country what he called "a fighting chance of emerging from this nightmare".
Meanwhile, the President of the Public Service Executive Union, Valerie Behan, said that neither a Yes nor a No vote was ideal, but that the continuation of members' very job security was at stake.
She urged members to focus on the core issue and to ignore all the extraneous "noise" that would emanate from both sides.
40% of voters undecided
A new opinion poll shows just under 40% of voters are undecided about how they will vote in the EU Fiscal Treaty Referendum at the end of May.
An Ipsos/MRBI poll in The Irish Times shows the Yes side ahead - by 58% to 42% - among voters who have made up their mind.
Voting in the referendum on the fiscal treaty will take place six weeks from today.
The poll on voting intentions was conducted among 1,000 voters around the country on Monday and Tuesday.
The results show that 30% said they would vote Yes, 23% would vote No, 39% are undecided, and 8% said they would not vote.
Since the last Irish Times poll on the issue last October, and before the treaty was agreed, the Yes side is up two points, the No side is down 24, and undecided and non-voters are up 22 points.
However, treaty opponents will note that the Yes side was even further ahead at the same point in the 2008 Lisbon referendum, which was rejected.
The American Chamber of Commerce has called for a Yes vote in the referendum.
The group, which represents over 600 companies in Ireland, says the treaty is crucial for investment and will allow Ireland to continue to access the European market.
Representatives of PayPal, IBM, Bank of America and Microsoft all attended a news conference today to launch their campaign.
Chamber President Peter O'Neill said Ireland had suffered significant reputational damage over the past 18 months and he said a Yes vote would ensure that Ireland's strategic interest would remain at the heart of the EU.
The Government launched its website to promote awareness of the referendum.
Every household is due to receive a copy of the treaty early next month, followed by an information note.