Irish Taoiseach Enda Kenny was among the early voters today as Ireland has its say on the European fiscal treaty.
About 3.1 million people have the right to vote in the only referendum being held across the EU on the controversial agreement to impose stricter budget controls.
The earliest indications of a result will not be until mid-morning tomorrow when the political tally men get to work with estimates from the 43 constituencies around the country.
And turnout is crucial with low voter numbers in two previous European referendums giving the anti-treaty side a huge boost.
Mr Kenny cast his ballot at St Patrick's National School in Castlebar, Co Mayo alongside his wife Fionnuala while his deputy prime minister Tanaiste Eamon Gilmore voted in Shankill, south Dublin this morning.
Elsewhere, Sinn Fein president Gerry Adams, one of the key figures in the anti-treaty camp, was also out early casting his ballot in the constituency of Louth where he moved to from Belfast to contest the Republic's 2011 general election.
Last night both Mr Kenny and Mr Adams made their final appeal for support.
The Taoiseach said a strong Yes would send a message that Ireland is on the road to the recovery and that it would help continue the strong flow of investment into businesses seen over the last few months.
"While there are still difficult challenges ahead, I hope people will vote Yes to continuing the progress we've made together," he said.
Mr Adams' final message to voters warned that the treaty would not solve the eurozone crisis and would put into the Constitution the failed austerity policies.
"I ask Irish citizens not to be bullied, not to give their democratic rights away, not to give up their say over Irish economic policy and not to write austerity into the Constitution," he said.
Polling stations in the 43 constituencies stay open until 10pm with counting due to begin at 9am tomorrow.
The Government has repeatedly insisted that, regardless of the result, Ireland will not be asked for its support a second time - a departure from the major Nice and Lisbon treaties of the last decade.
The big talking point as a broadcasting moratorium kicked in at lunchtime yesterday was a last-ditch High Court lawsuit brought by Sinn Fein against the independent advisory body, the Referendum Commission.
The party failed in its attempt to have the chair Judge Kevin Feeney retract remarks that Ireland would not have recourse to the proposed European Stability Mechanism (ESM) bailout fund unless the treaty was backed.
Three recent opinion polls have revealed a 60/40 split in favour of the European fiscal treaty among Irish voters but the No side are always boosted by lower voter turnout and whether the middle class vote can be mobilised is likely to determine the outcome.
The Taoiseach has said he was confident, but never over-confident, before polling day but his number two refused to make a call.
Labour Party Minister for Social Protection Joan Burton is the only senior figure to nail her colours to the mast and claim victory for the Yes side.
Ireland is the only country in Europe holding a referendum on the treaty as it is obliged to put major EU reforms to the public test, according to the Constitution.
Its record is unpredictable, having rejected the last two at the first vote only to accept the EU reforms in a re-run the following years. Regardless, the treaty will come into effect with the support of 12 states, with or without Irish support.
Voting has already taken place in some islands off the coast of Ireland - in Donegal voters cast ballots on Monday on Tory, Gola, Inishfree, Inishbofin and on Arranmore which, with 43 voters from an electorate of 173, had its lowest turnout ever.
The Mayo islanders polled on Tuesday while the Aran islanders and others off Connemara voted yesterday and the seven islands off the south-west of Co Cork vote with the rest of the country today.
A total 25 of the 27 European Union states have accepted the text of the treaty - with the exception of the UK and Czech Republic.
Only three states have ratified the treaty in full - Greece, Portugal and Slovenia - while Germany, Poland, Latvia, Romania, Austria and Denmark have begun the process.
If passed, the treaty will see stricter budgetary rules imposed on member states and penalties for those that fail to meet them.
The objective of the treaty is also to keep a control on deficits and ensure greater checks and balances are in place for money in and out of each country.
The Yes camp, including the Fine Gael, Labour and Fianna Fail parties, has argued that ratification will ensure Ireland has access to emergency funds from Europe should it require a second bailout.
Meanwhile, there was a security alert outside the constituency office of Sinn Fein's vice-president Mary Lou McDonald.
Army bomb experts were sent to the scene at North Strand Road, in north Dublin, just after 10am.
A Defence Forces spokesman said the scene was declared safe before midday and the device was removed to a military location for further examination.