Irish voters have rejected the European Union's Lisbon treaty in a referendum, the government acknowledged on Friday, potentially scuppering EU reform plans.

Justice Minister Dermot Ahern conceded the vote shortly after noon as tallies from around the country showed the treaty had been defeated in an overwhelming number of constituencies.

"It looks like this will be a 'No' vote," Ahern told RTE television. "At the end of the day for a myriad of reasons the people have spoken."