Vote-counting was due to start in Ireland at 0800 GMT Friday in an uncertain referendum to approve the replacement to the defunct EU constitution, known as the Lisbon Treaty, reported dpa.
Ireland, the only EU member to approve the treaty by popular vote, went to the polls Thursday in a referendum that is too close to call. Official results were expected by early Friday evening, but tallies should give an indication of the result by late morning.
Opinion polls ahead of the vote showed the no camp surging ahead to 35-39 per cent, while those backing yes were down to between 30-42 per cent, leaving around one-third of the electorate undecided.
Unofficial estimates put turnout in the low-to-mid 40-per-cent range, but the Irish Times put turnout at 50 per cent of the country's 3.1 million eligible voters.
A no vote would send the European Union project into confusion as 18 EU governments have already backed the treaty. The Lisbon Treaty could move forward without Ireland, but it was unsure how the country would work with the governments that have ratified it.
Irish voters have caused headaches for the EU in the past, rejecting the Nice Treaty in 2001 before approving it in a second referendum a year later.