All EU members must ratify the treaty for it to become law.
Ireland poses its greatest challenge, because all other EU nations are requiring approval only through their national governments.
So far 18 EU members have ratified it, including the parliaments of Estonia, Finland and Greece.
The Irish government, major opposition parties and business leaders all campaigned for a "yes" vote during a month-long campaign that emphasized Ireland's strong benefits from 35 years of EU membership.
In a final appeal on Wednesday, Jose Manuel Barroso, the European commission president said ratification by all states would allow the EU "to turn the institutional page and concentrate 100 per cent on delivering on the expectations of Europe's people."
But anti-establishment pressure groups from the far left and right mobilised opposition by claiming that the treaty's passage would result in Ireland losing control of everything from its business tax rates to its ban on abortion.
All the anti-treaty lobbyists argued that EU chiefs were trying to centralise power in Brussels by strengthening its senior positions and reducing the range of policy decisions that required unanimous support.
Voters at Dublin polling stations appeared evenly split on Thursday morning between the "yes" and "no" camps.
Most said they did not understand the treaty's implications well enough, and were essentially voting on whether they felt happy with Ireland's place in Europe.
"Ireland would still be the economic basket case of Europe without the EU. We should be doing everything we can to help EU institutions function better, because all the evidence shows they function in our interest," said a Padraig Walsh, an accountant and pro-treaty voter.
But many voters complained that the EU's near-doubling in size since 2004 had brought unwelcome change to Ireland, particularly the large influx of job-seekers from Poland and the Baltic states who fill in the majority of available jobs.
"I feel like a foreigner in my own land. There's been too much change, too quick," said Eugene Leary, a laid-off construction worker who has turned to part-time taxi work to make ends meet.
"You don't mean to be a bigot or a racist. But you would like to see your country keep control of its identity, and make sure your own people are being looked after first. That's just not happening."
Many "no" voters said they were annoyed that the Lisbon Treaty contains largely the same reform goals as the rejected constitution, and expressed solidarity with the voters of France and the Netherlands who opposed that document.
"I think part of being a good European is respecting the votes of the people," said Niall Kavanagh, a lawyer who said he had voted "yes" to previous EU treaties but voted "no" this time because EU chiefs appeared to be trying to get around the votes of the French and Dutch.
"How many times do people have to vote no before Brussels respects the outcome?" he said, noting that Ireland rejected a previous EU treaty in a 2001 referendum, only to be asked to vote again two years later.
"Somehow we have to create an EU where no really means no."
Results from the poll are expected on Friday.