With the successful introduction of its new single currency, the euro, the European Union is gearing up for an even more daunting challenge in the years ahead - bringing up to 10 mainly former communist countries into the prosperous 15-nation club. But the bloc's enlargement, now tentatively scheduled for 2004, could be derailed by voters in one of its smallest member countries.

There are all sorts of potential obstacles to bringing about the EU's scheduled expansion. Greece, for instance, has made it clear that if Cyprus, or at least the Greek-speaking part of the island, is not among the first batch of new entrants, Athens will veto the other candidates.

Germany has been hinting that countries such as Estonia, Hungary, and Slovenia, considered to be among the front-runners for membership, should wait until Poland, which is judged to be a laggard in its efforts to meet EU requirements, is ready to join.

And France, worried that its influence within the EU will dwindle as the center of gravity of an enlarged bloc moves eastward, is also quietly suggesting that perhaps the 10 candidates should wait until Bulgaria and Romania are ready to join in 2006 or later.

But enlargement faces another potential roadblock in one of the most unlikely places - Ireland.

Ireland is one of the few EU countries that requires its voters to approve major EU treaties in a referendum. Last June, to the surprise of many Europeans, Irish voters rejected the Treaty of Nice, which would reform EU institutions and allow them to accept new members. True, the turnout was small, but those that bothered to vote said "no". And that embarrassed the Irish government and its partners in the EU.

Irish Foreign Minister Brian Cowan says the treaty will lapse and the EU enlargement will be at risk if Irish voters do not change their minds in another referendum this year.

"It posed a dilemma for us because, if the 15 members do not ratify the Treaty of Nice by the end of the year, there is a very, very major problem. We do not believe that it is in our national interest that Ireland put itself in that position," he said.

Irish politicians are still scratching their heads to try to figure out why voters said "no" to the treaty. The Irish, who have benefited immensely from EU membership, are among the most pro-European people in the Union.

Maurice Hayes, the chairman of Ireland's National Forum for Europe, which was set up to stimulate debate on the question, suggests that voters were concerned Ireland might lose its traditional neutrality as the EU moves toward creating its own military rapid reaction force.

"A main concern, actually, is the whole implication of neutrality and foreign policy and the rapid reaction force. That sort of thing. Fear of sort of being dragooned into somebody else's foreign wars. There is a fear of a small country being submerged in a huge conglomerate," he said.

But Foreign Minister Cowan says that assessment is not correct.

"What the European Union is trying to do is to get involved in peacekeeping, involved in conflict resolution, conflict prevention. It is very much in line with our foreign policy traditions, anyway," he said.

What both men agree on is that Irish voters ignored the advice of major political parties, labor unions, employer federations and even the Catholic church, all of which urged a "yes" vote in the referendum.

Enlargement cannot take place until the 15 EU members endorse the Treaty of Nice. Most experts say if a new referendum is held in Ireland during a general election, which is due by mid-year, the treaty changes would be approved, because a high turnout would ensure a more representative sample of the pro-European electorate. But, in a one-off vote, with another low turnout, the experts fear that voters will say "no" once again.

That kind of result would not only set back enlargement. It would also undermine Ireland's standing in the European Union and among the countries that want to join.