With only partial results in, electoral officials said "yes" votes were outnumbering "no" in 41 of Ireland's 43 constituencies - a total reversal from the country's stunning rejection of the Lisbon Treaty last year.
RTE radio said early tallies from counting centres showed constituencies such as Dublin Central and Dublin North East had voted 56 per cent in favour, while in Galway city early indications put the "Yes" vote as high as 63 per cent.
Micheal Martin, the Irish foreign minister, hailed the result.
"I am delighted for the country. It looks like a convincing win for the 'Yes' side on this occasion," he told national radio.
The Irish government called the second referendum under pressure from EU leaders and the executive European Commission in Brussels.
The treaty cannot take effect unless all the member states ratify it.
Richard Greene, a spokesman for the opposition Coir group, criticised the decision to put the decision to a second vote.
"It looks like a 'Yes' vote. I want to sympathise and commiserate with all our people who put in a great effort for the love of their country," he said.
"We are extremely disappointed that the voice of the people was not heard the first time around."
Campaigning before the vote brought warnings from celebrities, politicians and business leaders that a second "No" would harm Ireland's position as it battles recession.
Brian Lenihan, the finance minister, said that the swing in support for the treaty was likely a response the dire economic situation.
"We face very serious [economic] challenges," he told RTE.
"The government is not engaged in any celebratory parties. We are in a very difficult place and that's precisely why people have voted 'yes'"
Irish finance minister
"The government is not engaged in any celebratory parties. We are in a very difficult place and that's precisely why people have voted 'yes'."
Dublin received assurances from EU partners that its sovereignty on issues like taxation, military neutrality and abortion would not be affected by adopting the Treaty of Lisbon.
Only three countries, the Czech Republic, Ireland and Poland, have still to approve the treaty. Germany ratified the treaty last Friday after Horst Koehler, the president, signed the final ratification act.
Vaclav Klaus and Lech Kaczynski, the Czech and Polish presidents, had both said they would not sign for their respective countries until Ireland has agreed to back the treaty.
Prominent changes envisaged include more qualified majority voting in the council of ministers, increased involvement of the European parliament in the legislative process and the creation of a president of the European Council with a term of two and half years.
It would also create a High Representative for Foreign Affairs to present a united position on EU policies.
If ratified, the treaty would also make the Union's human rights charter, the Charter of Fundamental Rights, legally binding.
Opponents of the Treaty of Lisbon have argued that the treaty will centralise the EU and weaken democracy by moving power away from national electorates.
Initial attempts at creating an EU-wide constitution have met with failure. The first, the European constitution, failed in 2005 due to rejection by French and Dutch voters.
The Treaty of Lisbon, the constitution's replacement, was originally intended to have been ratified by all member states by the end of 2008, so it could come into force before the 2009 European elections.
However, the rejection of the treaty on June 12, 2008 by Irish voters means that the treaty cannot currently come into force.