Polls opened at 7 a.m. (0600GMT) in a referendum on the 2006 treaty, a painstakingly negotiated blueprint for reforming how the 27-nation bloc makes decisions and presents itself to the wider world. Results come Saturday.
Ireland is the only EU member requiring the treaty to win majority approval from voters. The Irish rejected it last year, but are voting again after EU leaders reaffirmed their military neutrality, control over tax policies and right to keep abortion outlawed in this predominantly Catholic country. The treaty can't become EU law unless Ireland approves.
In eve-of-poll declarations, Prime Minister Brian Cowen and opposition leaders alike said their party workers would lobby the public all day to vote yes. They said a second "no" would do the most damage to Ireland itself, because the country is in a deep recession and requires European Central Bank support to revive its banks and combat a runaway deficit.
"With a `yes' vote, Ireland will retain the confidence that it is a positive and influential member of the union, and the union will be allowed to move forward to tackle urgent problems," Cowen told voters. "With a `no' vote, confidence in Ireland will inevitably suffer."
The left-of-center Labor Party urged its supporters to back the treaty _ even though rejection would likely topple its opponents in Cowen's 17-month-old government. Labor leader Eamon Gilmore said Ireland needed to demonstrate its pro-EU credentials now, and could force Cowen from power later.
Gilmore cautioned voters not to stay at home as opinion polls suggested a strong victory for the pro-treaty side. He noted that 53.4 percent of voters said "no" in June 2008 _ but on just a 53 percent turnout _ after polls had indicated the treaty's approval.
"The greatest mistake that could be made by those who value our membership of the EU would be to assume that the referendum is going to be passed," Gilmore said.
Ireland's major newspapers and broadcasters said they planned no exit polling, leaving the outcome unclear until Saturday.
The Irish Times, in its lead editorial Friday, didn't explicitly recommend to readers how to vote on what it called "an imperfect, technical treaty that even its strongest supporters find difficult to love."
The Lisbon Treaty proposes to make it easier for European summits to reach policy decisions by majority rather than unanimous votes; create new posts of president and foreign minister for promoting EU policies on the world stage; and give both national legislatures and the European Parliament more say in shaping and approving policies.
Treaty opponents emphasize their view that the EU is undemocratic and seeking greater powers to impose unpopular policies on Ireland, including higher taxes, lower wages, legalized abortion and euthanasia, and increased immigration. A government-appointed commission tasked with explaining the treaty insists all these claims are baseless.
One major difference between the 2008 campaign and the current one is the fate of Ireland's member of the European Commission, the bloc's executive branch.
The original treaty sought to reduce the commission size by restricting the representation of Ireland and other smaller nations to 10 of every 15 years. After many Irish voters cited this as their main reason for voting against it, EU chiefs relented _ meaning that a "yes" vote this time will safeguard Ireland's seat at the ministerial table.
On the Net:
Lisbon Treaty search engine, http://searchablelisbon.org/index.html
Referendum Commission guide, http://www.lisbontreaty2009.ie/
Ireland for Europe pro-treaty group, http://www.irelandforeurope.ie/
Conservative anti-treaty grup Coir, http://www.coircampaign.org/
Socialist Party opponent of treaty, http://www.joehiggins.eu/
Ryanair "yes" campaign, http://tinyurl.com/y9mg4cx