Nearly three million people will be entitled to cast their verdict on the Nice Treaty today when polling stations open at 9 a.m. for 12 hours of voting.
The first figures from seven constituencies, which are voting electronically, will be available in the early hours of Sunday morning, and should offer trends for the final result.
Predicting cold, dry weather for most parts of the country, Met Office forecaster, Ms Sarah O'Reilly said: "The weather can't be blamed if people don't vote." Voters are asked to bring along some form of identification when they go to polling stations, such as a passport, photo ID, a bank book, or marriage certificate.
Electronic voting and counting will be used in the Dublin Mid West, Dublin North, Dublin West, Dublin South, Dublin South West, Dún Laoghaire and Meath constituencies.
Counting for the Dublin electronic constituencies will begin after polls close at 9 p.m. in the RDS, while the Meath ballot will be counted in the Courthouse, Trim. Counting in the remaining 35 constituencies will begin at 9 a.m. on Sunday and the result will be announced from Dublin Castle, probably by 4 p.m.
Voters will be asked to offer a Yes, or No to the following single question: The 26th Amendment of the Constitution Bill, 2002, proposes to insert the following subsections after subsection 6 of section 4 of Article 29 of the Constitution.
7. The State may ratify the Treaty of Nice amending the Treaty on European Union, the Treaties establishing the European Communities and certain related Acts signed at Nice on the 26th day of February, 2001.
8. The State may exercise the options or discretions provided by or under Articles 1.6, 1.9, 1.11, 1.12, 1.13, 2.1 of the Treaty referred to in subsection 7 of this section but any such exercise shall be subject to the prior approval of both Houses of the Oireachtas.
9. The State shall not adopt a decision taken by the European Council to establish a common defence pursuant to Article 1.2 of the Treaty referred to in subsection 7 of this section where that common defence would include the State.