The referendum, Ireland's third on Europe in four years, puts it back in the spotlight after it avoided much of the recent heat from the eurozone's debt crisis by dutifully implementing its 85 billion euro EU/IMF bailout.
The ‘fiscal compact’ treaty for stricter budget rules needs the approval of only 12 of the 17 euro zone countries to be ratified. Therefore the Irish voters cannot derail this new intergovernmental treaty (see background). But certainly, an Irish rejection would undermine one of Europe's key initiatives just as problems mount in Spain and Greece.
The Irish debate has been squarely framed around a clause in the treaty stating that only those who sign up can access future European bailout money.
The government-led 'Yes' campaign has warned of unpaid wages, empty cash machines and a sudden halt to inward investment if the treaty is rejected.
The 'No' camp, spearheaded by the rising Sinn Fein party and a small number of trade unions, say Europe will not dare cut Ireland off if, as is likely, it needs further official funding when its current bailout runs out next year.
However, Ireland's two-year borrowing costs rose above its 10-year bond yields for the first time since January on Thursday on fears that a shock 'No' vote would threaten its access to funding.
"What is at stake is the future of the country," Maire Geoghegan-Quinn, the European Commissioner in charge of research and science, and Ireland's most senior official at the executive, told Reuters.
"It is probably the single most important referendum that we have ever had in the country. It is about ensuring that for industry there is a landscape which has certainty. 'Yes' equals certainty. 'No' equals no-man's-land."
Turnout key
Every poll taken since the vote was called in February indicates it is likely to pass by a margin of around three to two, once undecided voters are excluded. This suggests that the referendum has not been turned into a vote against the unpopular austerity policies.
However, with up to 20% of voters yet to make up their minds according to some polls, and figures on Wednesday showing that unemployment remains at a stubbornly high 14%, the government cannot completely discount encountering an angry electorate at the ballot box.
As has been the case in previous European referendums, the result may rest on how many people turn out to vote.
Ireland has twice rejected European treaties in the past 11 years before reversing course in repeat votes that had a higher turnout.
The available opinion polls give no reliable indication of how many people may turn up to vote.
The EU's Nice Treaty was rejected in 2001 on a 35% before being approved the next year, when 49% voted.
The Lisbon Treaty was rejected on a 53% turnout in 2008, then approved the following year on a turnout of 59%.
"It's hard to say at what point the 'No' vote takes over but definitely the 'Yes' side would want to be heading towards (a minimum turnout of) 50%," said Theresa Reidy, a politics lecturer at University College Cork.
Prime Minister Enda Kenny's Fine Gael party urged its members on Thursday to "contact friends, work colleagues and family members" and ask them to cast their votes in favour of the treaty.
"It really does look like it is going to go through, but the big caveat is that turnout is going to be very important. If turnout were to slip, that would be very alarming for the 'Yes' side," Reidy said.
