Ireland will hold its referendum on the European Union's new fiscal treaty on May 31, the government said on Tuesday, in what will probably be the only popular vote in the bloc on plans for stricter budget discipline.

Ireland joined 24 other European Union states in January in backing the treaty, which its proponents say is necessary to restore confidence in the euro zone and tame the debt crisis that has sucked in Greece, Ireland and Portugal and now threatens the much bigger Italian and Spanish economies.

After taking legal advice, Prime Minister Enda Kenny last month said Ireland would hold a referendum on ratification and early opinion polls have given supporters of the fiscal compact a clear lead.

This is a treaty on stability and is about ensuring long-term stability, recovery, growth and jobs, Deputy Prime Minister Eamon Gilmore said after announcing the date.

The bill to amend the constitution will be published towards the end of this week, he added.

While Ireland's initial rejection delayed the implementation of the EU's Lisbon Treaty in 2008, the fiscal pact does not require EU-wide ratification and Ireland would simply drop out were the referendum to reject it.

The government has until the end of the year to ratify it, but chose a date 10 weeks away in an apparent attempt to take advantage of an early lead in the polls.

A poll on Saturday showed that 49 percent would vote in favour of the treaty with 33 percent opposed and 18 percent still to make up their minds.

The government is also likely to strike a deal with the European Central Bank (ECB) this week to refinance a portion of the country's bank debt, a move that might placate some opponents of the treaty.

It's only now that you'll see the proper campaign start and get a better of idea of whether they are really that far ahead, said Eoin O'Malley, a politics lecturer at Dublin City University (DCU).

It's not something like abortion where you've got a strong opinion one way or another, so the campaign could be important in this case, he added, referring to 1992 and 2002 referendums.

Ireland's main opposition party Fianna Fail has said it will join the governing Fine Gael and Labour parties in campaigning for a 'Yes' vote on the fiscal pact.

A poorly communicated campaign contributed to the defeat of a government-supported referendum on parliamentary committees last October, and Fianna Fail leader Micheal Martin urged the government to mount a comprehensive information campaign about the EU pact.

Sinn Fein leader Gerry Adams, whose party forms the second largest opposition bloc in the lower house and is riding a wave of popularity on its anti-austerity stance, confirmed his deputies would campaign against the treaty.

(Editing by Ruth Pitchford)