Taoiseach Leo Varadkar, who called the referendum, was among the first to cast his ballot when polling stations opened.
He said: "I always get a little buzz from voting. It always feels like democracy in action.
:: Ireland's abortion dilemma: A nation divided
"I'm not taking anything for granted of course, but quietly confident there's been a good turnout so far across the country and hopefully it will be a yes vote."
In some places, turnout was reported to be higher than for the vote on same-sex marriage three years ago.
'Home to vote' campaigns had gained momentum in recent weeks, bringing hundreds of young people back to exercise their democratic right.
With generations of women having left Ireland to access abortion, many felt it was time to change the direction of travel.
Emily Arnold, who flew in from Barcelona, told Sky News: "A short trip home for me so that others don't need to travel for healthcare... I mean, it's a no-brainer."
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Others made the journey to "Save the Eighth" - an amendment to the constitution, equating the life of the unborn child with the life of the mother.
If the constitutional ban is lifted, Ireland's government is committed to introducing unrestricted access to abortion for women up to 12 weeks pregnant.
Sarah Gillespie travelled home to Donegal from Pennsylvania in the US to vote 'no'.
"We voted in the equal marriage referendum in 2015, which I voted for and completed supported, and I think that shows we are progressing," she said.
"That's kind of why there is a divide. Young people may think that this is progressive whereas I disagree. I think voting 'no' is still progressive for Ireland."
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In the space of 40 years, Ireland has legalised contraception, divorce and same-sex marriage, but abortion has always been the most emotive issue.
After months of political debate and an eight-week long campaign, Mr Varadkar had described the vote as a "once in a generation" opportunity.
