Voting's underway in the proposed Thirtieth Amendment to the Constitution.
Polls will close at 10 o'clock tonight and an indication of the result will be known by mid-morning tomorrow.
In all 3.13 million people are eligible to vote in this referendum on the Treaty on Stability, Coordination and Governance in the Economic and Monetary Union.
Hundreds of polling stations in all 43 constituencies will remain open until 10pm tonight.
You should have received a polling card telling you where to vote, but you don't need it to cast a ballot - voters are encouraged to bring along some form of photo ID.
All voters are encouraged to use their democratic right and go out and vote - remember just one vote could decide the outcome.
At 10 tonight the ballot boxes will be sealed and transferred to 26 count centres across the country.
Counting gets underway at 9 o'clock tomorrow with a good indication of the result expected later in the morning.