Delegates at the party’s Ard Fheis voted by three to one to back a motion from Kildare North constituency urging the party “to oppose any attempt to diminish the constitutional rights of the unborn”.
A second motion from the party’s London branch – backing a “woman’s right to choose” was also heavily defeated.
The result comes as a special cross-party committee of TDs and Senators is examining the controversial issue.
The committee is considering a report by the Citizens’ Assembly which recommended radical change to the 1983 amendment which guarantees the right to life of the unborn child while giving equal importance to the life of the mother.
The party leader Micheál Martin’s call for a “respectful discussion” was for the most part respected during a two-hour debate on health issues which was dominated by the abortion resolutions.
Former Senator John Hanafin urged no change to the constitution.
He said he was an adoptive father of three children and he noted that adoption agencies were now closing in Ireland.
Mr Hanafin also demanded “fair play in the media” for the forthcoming debate.
Tadhg O’Donovan from Leixlip said he believed the “pro-life” lobby was not getting fair play in the Oireachtas committee and in the Citizens’ Assembly.
Audrey Eager from the London branch said divorce had not devalued marriage and giving a woman the right to make her own decisions would not devalue human life.
She pointed out that 4,000 Irish women travel to Britain each year to have an abortion.
But Róisín Bradley from Donegal said the Eighth Amendment had a hugely positive effect and society should build upon its gains.
Dun Laoghaire-Rathdown Cllr Kate Feeney said the Eighth Amendment had not stopped Irish women travelling to England for an abortion and risking their health.
Her County Council colleague Jennifer Cuffe also referred to the numbers travelling to London saying: “it’s happening – we cannot turn our backs on it anymore”.
Taoiseach Leo Varadkar has already signalled that the government intends to hold a referendum on abortion next year.