The EU's other 14 members need only a parliamentary vote to ratify the treaty, negotiated in Nice last December, which prepares the way for admitting 12 new members. The Irish constitution, however, insists on a referendum.
Only two weeks before the vote Irish opinion polls showed a three-to-two majority in favor of the Nice Treaty - but the "yes" voters stayed home in droves.
"The outcome of a referendum in one country cannot block the EU's most important project," said Guenter Verheugen, the EU Commissioner for Enlargement, as near-panic surged through the former communist countries of Eastern Europe, which base their hopes of prosperity on early membership in the EU.
But he is wrong. A narrow majority of Irish voters on a 32 percent turn-out - only 76,000 people, in fact - can stop the expansion.
"This is payback time for us," said Bertie Ahern, Ireland's prime minister, at the start of the referendum campaign. "This is our chance to show that generosity of spirit that was shown to us so often in the past."
He was getting right to the heart of the matter, for in Ireland the campaign was not really about the Nice Treaty, which merely streamlines EU institutions. It was more about Ireland losing its lavish EU subsidies.
When Ireland joined in 1973, the average Irish income was two-thirds of the European average. So Ireland got lavish grants from the EU budget, amounting over the years to around $6,000 per capita. Partly as a result of that, it now has a booming economy where incomes are higher than the European average.
Ireland's transfer payments are due to stop in 2006, by which time the EU will need all the subsidy money available to help the new, much poorer members in Eastern Europe.
This cut is not due to the Nice Treaty, but it certainly is a consequence of expansion, and there were just not enough Irish voters who believed the generosity that was once shown to them should now be shown to Poles and Bulgarians. Other issues also exercised the motley "no" coalition, but few of them were really about the treaty either. Some feared that the proposed European Rapid Reaction Force would compromise Ireland's neutrality.
Others worried that the European Charter of Fundamental Human Rights could lead to a successful challenge to Ireland's law banning abortion. And the majority who didn't buy those arguments didn't bother to vote.
Most of the Irish, on their neutral island out in the Atlantic, have little emotional commitment to the concept of a united Europe, and see the EU as little more than a cash cow.
If it is now failing to deliver, then some means must be found to squeeze more out of it.
The "no" vote is an effective form of blackmail, for Ireland's veto can keep over 100 million people in 12 other countries waiting indefinitely.
So various bribes and assurances will be offered to Ireland - maybe some more money, maybe a guarantee that its ban on abortion is safe from the courts - and then Prime Minister Ahern will use that as an excuse to call a second referendum, probably before the end of this year.
It may work, it may not.