The voting is being watched throughout Europe. Ireland is the only country to put to the EU plan, instigated by Germany early this year, to a public vote. Irish constitutional law requires public approval on major reforms.
The treaty calls on member states to limit spending and stick to budgetary targets. It aims to coordinate EU fiscal and budget policies and hold annual structural deficits within 0.5% of gross domestic product, with bailout funds from the European Stability Mechanism available to those who ratify.
Fines are to be imposed on those who fail to comply with debt targets.
Of the 27 EU nations, Britain and the Czech Republic have opted out of the treaty, which must be ratified by 12 of the 17 Eurozone states by next March. So far, Romania, Slovenia, Portugal and Greece have approved it.
Polls indicate that the treaty will pass, but Ireland has a quixotic record on EU treaties. Two previous referendums -- on EU enlargement in 2001 and a more streamlined EU administration in 2008 -- were rejected by Irish voters, then accepted in a second vote after amendments. This time there will be no second vote for Ireland.
But Irish voters are expected to reluctantly favor more austerity and continued access to EU rescue funding. Ireland has already been saved by a massive European bailout of $108 billion in 2010.
Although one poll shows the treaty passing with 60% support, there is also the specter of low turnout.
In a last appeal before the balloting, Prime Minister Enda Kenny urged Ireland's 3.1 million voters to "vote yes on Thursday, yes to stability, yes to investment," and give greater credibility to Ireland when it takes over the EU presidency in January.
Fierce criticism of the measure has come from the Sinn Fein opposition party, once the political arm of the outlawed IRA revolutionaries of Northern Ireland but now a legitimate and popular left-wing force in Northern Ireland and the Irish republic. Party leader Gerry Adams told voters not to give up their say over Irish economic policy and "not to write austerity into the Constitution."
Results of the vote are expected to be known by Friday.
-- Janet Stobart
Photo: Posters for and against the EU fiscal treaty are seen outside government buildings in Dublin, Ireland. Credit: Peter Morrison / Associated Press