The Taoiseach Mr Ahern and the President Mrs Mary MacAleese were amongst the high profile voters casting their ballots on the second Nice Treaty Referendum today.
Mr Ahern, who has promised his fellow European leaders a Yes result on the Nice treaty on enlargement, is calling the referendum nothing less than "a date with history."
Ten nations in eastern Europe and the Mediterranean which hope to join Europe's exclusive club of rich states in 2004 are closely watching the outcome, as Irish voters rejected the treaty in an earlier referendum last year.
Mr Ahern, who has led the campaign for a Yes, cast his vote Saturday afternoon in a polling booth set up in a primary school in Dublin. "I voted Yes because it's the right thing to do in my view," he said.
The President Mrs MacAleese also cast her vote shortly after polling stations opened after 9 a.m., a spokesman said.
The polls will remain open for 12 hours. The results from the seven constituencies that are voting electronically will be known around midnight with final results confirmed tomorrow evening.
Turnout so far was reported to be higher in Dublin than in the rest of the country.
European Commission President Romano Prodi insisted today that there is no back-up plan in case of a second rejection of the Treaty by the Irish, as EU leaders have long warned.
But observers argue that this stance is partly political strategy, designed to sway Irish waverers away from the No vote. Experts say a No vote would almost certain push enlargement back to 2006, but would not be catastrophic.
AFP