No referendum can change the moral truth that abortion is wrong, Bishop of Ferns Denis Brennan has said.
The direct and intentional killing of an unborn human would be just as immoral the day after it was legalised as it had been the day before, Bishop Brennan added.
Draft legislation to be introduced if the referendum on repealing the Eighth Amendment is passed proposes unrestricted terminations be made available to women who are up to 12 weeks pregnant.
The bishop said: "What repeal would mean is very clear, namely that the unborn boy or girl whose heart beats at 21 days, and the older unborn baby who has all of her/his vital organs at 12 weeks, will have no rights at all in Irish law, should people vote yes to repeal.
"This 12-week-old unborn baby, who is now enjoying for the first time the ability to kick, to move and to yawn, would, in the first stretches of young life, be without the basic protection of the right to life itself.
The poll is due to be held on May 25.
Taoiseach Leo Varadkar launched Fine Gael's campaign for Yes on Saturday by characterising it as the compassionate answer for women with crisis pregnancies forced to travel abroad for abortions.
The bishop said: "No referendum can change moral truth.
"The direct and intentional killing, of an unborn human being, would be just as immoral the day after it was legalised, as it had been, the day before.
"That any person, at any age, would have no rights at all is not, I believe, what a majority of Irish people want.
"This is what repeal proposes and will come to mean were it to pass.
"As voters, we are the unborn baby's last line of defence.
"I ask that you weigh carefully this responsibility and act in the best interest of the unborn child."
Abortion is currently only available when a mother's life is at risk, but not in cases of fatal foetal abnormality, rape or incest.
Mr Varadkar has said next month's referendum could represent a coming-of-age moment when the nation stops cold-shouldering those in crisis.