Sinn Féín delegates have adopted a policy to campaign for abortion in cases of fatal foetal abnormality.
The motion, backed by the party's leadership and central council, was endorsed by a large majority of delegates at the party's Ard Fheis in Derry.
The adoption of the motion meant that another motion, seeking the blanket right to abortion wherever a woman seeks one, automatically fell.
A similar motion asking Sinn Féin to maintain the status quo - where abortion is allowed only when the mother's life is at risk - also lapsed.
However delegates adopted a separate motion calling for the repeal of the 8th Amendment to the Constitution of Ireland, which equates the lifes of the mother and unborn child and which restricts the circumstances under which abortion is permissible.
Mary Lou McDonald said that allowing women and couples, whose unborn children suffered from fatal foetal abnormality, was the "decent" and "republican" thing to do.
She said those women and couples should be shown compassion and care, "but above all, choice".
The discussion on abortion was among the most divisive of those taking place at the two-day conference in Derry's Millennium Forum.
One delegate from Cork said unborn children suffering from fatal foetal abnormalities deserved to be allowed to live for as long as their conditions allowed.
Another said women would only be truly liberated when they had full discretion over their right to carry a child.
Debate ongoing - these are the four abortion motions #SFAF2015 delegates will be asked to vote on pic.twitter.com/ayaYH6dLvw
— Gavan Reilly (@gavreilly) March 7, 2015
TD for Meath West, Peadar Toibín, had been prepared to speak in opposition to motion 122, the motion backed by party leaders endorsing access to abortion in cases of fatal foetal abnormality.
However he did not make it to the podium in the allowed time for speakers, and resumed his seat some time before the vote was scheduled to be called.
Deputy Toibín - who was suspended from the party for six months when he voted against party lines on the Protection of Life During Pregnancy Bill in 2013 - had pledged to vote against the motion.
