"You are talking about a coat that was knitted together by 27 different countries," Lenihan said after casting his vote in Dublin. "It is very difficult to knit that coat again."
Fourteen countries have already ratified the treaty in their national parliaments. The treaty is due to come into force on January 1 if all nations ratify it.
EU leaders fear some countries such as Britain may suspend the process if Ireland votes "No". A senior EU diplomat said Britain had told its EU allies it had "no intention to pull the plug on this" even if the Irish vote no.
The "Yes" camp says Ireland's diplomatic clout and economy would suffer if voters rejected reforms drawn up by a union whose support underpinned the "Celtic Tiger" economic boom.
ECONOMIC CHALLENGES
Data on Thursday showed shoppers reining in spending. Unemployment is rising, although below the European average.
"We face economic challenges and Europe provides a framework of stability for investment in Ireland. If you create uncertainty in that framework, that is bad for Ireland and bad for Europe," said Lenihan.
A "No" vote could dent the image of a pro-European, English-speaking euro zone country, although economists do not predict grave long-term consequences.
Voting before heading into work, 45-year-old John Devlin said he expected the vote to be "very tight" but he hoped and believed it would be carried.
"Ireland was a dusty backwater until we joined the European Union," Devlin said. "I think in Europe we should be united and seen as a counterbalance in global affairs."
Opponents such as nationalist party Sinn Fein say Brussels and the government have tried to bully people into backing the treaty and say it should be renegotiated to better protect Ireland's sovereignty, military neutrality and influence.
"The "Yes" campaign believe they have this in the bag," said Mary Lou McDonald, a Sinn Fein member of the EU parliament. "It will depend on turnout. I think it will be a close call."
Low turnout was a factor when Ireland almost scuppered EU plans for eastwards expansion by rejecting the Nice treaty in a 2001 referendum where only 35 percent of the electorate voted. That was passed in a second vote criticised as undemocratic.
