KEY POINTS:
The fate of an arduously-crafted pact to overhaul the European Union's institutions lies in the hands of three million Irish electors on Friday, and just a tiny number of votes could determine the outcome.
EU officials are biting their finger nails down to the quick, after one opinion
poll suggested the treaty would be rejected by five percentage points but another suggested it could squeak through by three points. Almost a fifth of the electorate is undecided.
The referendum is the latest twist in a three-year saga that has placed the political future of the EU and its half a billion citizens on hold.
If the "No" vote carries the day, no one knows what the next step will be.
"There is no Plan B," European Commission chief Jose Manuel Barroso warned last month. "If there was a "No" in Ireland or in another country it would have a very negative effect for the EU."
"If the outcome is a 'No' then it's back to the drawing board," an EU source said in Brussels.
The vote focuses on whether to endorse the Lisbon Treaty, under which decision-making within the EU would be streamlined in order to accommodate its expansion from 15 to 27 states. It would also create a foreign policy chief and a full-time EU president.
For it to take effect, it must ratified by all EU states, a goal that France, which holds the EU presidency for six months from July 1, has vowed to achieve by year's end.
So far 15 have completed or nearly completed this, through parliamentary procedure.
Ireland alone has to hold a plebiscite, as the treaty entails changes to its constitution. To complicate things, the treaty carries a lot of baggage in Ireland - and its public have a reputation of contrarian voting.
The pro-Europe camp is led by fledgling prime minister Brian Cowen and has the backing of all three major parties in the lower house of parliament. It argues Ireland owes its fabulous growth in wealth thanks to subsidies from and trade with Europe.
A 'No' vote would be viewed as gross ingratitude and cripple Ireland's influence within the EU.
"We've done astonishing well from Europe," said Ireland's minister for Europe, Dick Roche.
"We're saying to people: don't throw that away. Express your vote as a positive affirmation for Ireland's place in Europe."
French Foreign Minister Bernard Kouchner warned Europe would react with "enormous incomprehension" if a 'No' vote won.
"The first victims would be the Irish. They have benefited more than others," Kouchner told France's RTL radio.
Irish Eurosceptics, though, are saying that such remarks are just scare tactics to ram through a repackaging of what they consider to be an assault on national sovereignty - the draft European Constitution ditched by French and Dutch voters in 2005.
Analysts say the No camp is being helped by worries over Ireland's economy and the mind-numbing dullness of the Lisbon Treaty itself.
The document is extremely technical and 346 pages long, making it "more Stephen Hawking than J. K. Rowling," in the words of Defence Minister Willie O'Dea. Irish EU Commissioner Charlie McCreevy last month admitted he didn't expect "any sane and sensible person" to read it, the Irish Independent reported last month.
In June 2001 Irish votes rejected the draft Nice Treaty that set down institutional reforms enabling the "Big Bang" membership expansion to go ahead.
But this was reversed in October 2002 after European leaders, addressing one of the voters' concerns about the treaty, formally recognised Ireland's long-standing military neutrality.
This time around, European leaders have kept their heads down, warned by Dublin not to intervene too fervently in favour of the treaty.
The vocal - and in several countries highly influential - anti-EU lobby is hoping fervently for a rejection.
Yesterday, a wealthy British business, Stuart Wheeler, began a High Court challenge in London to press Britain to follow Ireland's example.
If Lisbon is tossed out by Ireland, the EU will stumble along using the Nice Treaty until a solution is found.
But critics warn Nice will lead to decision-making deadlock as countries wield their right of veto.