Younger voters showed more support for overturning the amendment than older voters, according to RTÉ exit poll projections.
Friday's vote repealed the Eighth Amendment to Ireland's Constitution, which was added in 1983 and granted equal rights to unborn fetuses and amounted to an nearly complete ban on abortions. Irish women who want abortions now must to travel overseas to have them.
Official counting for Friday's referendum on whether or not to liberalize Ireland's abortion laws was still under way, and final results are not expected until Saturday afternoon.
The referendum's passage is a huge win for reproductive-rights advocates in a Catholic country that for decades pledged to give women and fetuses an "equal right to life" and made abortion punishable by up to 14 years in prison.
Anti-abortion groups in Ireland vowed to continue their fight to protect existing abortion laws and the rights of the unborn child despite an apparent referendum setback.
In 2012, an Indian woman called Savita Halappanavar died in a Galway hospital after being refused an abortion.
Ireland voted by a landslide to ditch its strict abortion laws in a landmark referendum hailed by Prime Minister Leo Varadkar who said he wanted the law in force by the end of the year.
Since then, various anti-abortion and pro-choice groups have campaigned for the Eighth Amendment to be repealed.
That set the trend for the day, as constituency after constituency voted in favour of change, peaking with 78.5 per cent of voters in Dublin Bay South voting Yes.
In 2015 the country voted overwhelmingly to legalise same-sex marriage in a historic referendum.
The Irish government is planning to bring legislation before the Dáil, providing for abortion on request up to the 12th week of pregnancy, with a three-day "cooling off" period before abortion medication is administered.
One of Ireland's leading anti-abortion groups says the abortion referendum result is a "tragedy of historic proportions" in a statement that all but admits defeat in the historic vote.
He said the government will now move quickly to legalize abortion. Abortion on demand would deal Ireland a tragic blow but the pro-life movement will rise to any challenge it faces.
"But she said "I'm sorry, unfortunately it's a Catholic country" and it's the law that they can't abort when the foetus is live".
People over 65 voted 60 percent against repealing the current legislation, which only allows terminations in cases where the mother's life is in danger.