Archbishop Eamon Martin has issued a pastoral message ahead of Friday's referendum.
In the address, the Archbishop of Armagh urges voters to "think of two lives".
"When I wrote to you in January about protecting the Eighth Amendment, I invited you to spread the word about the precious gift of life from the first moment of conception until the moment of natural death. I encouraged you to be 'missionaries for the cause of life'.
"Since then the Supreme Court has clarified that if the Eighth Amendment is repealed, unborn children in Ireland will have absolutely no constitutional rights.
"A vote to repeal the Eighth Amendment on 25 May would therefore pave the way for a very liberal abortion regime in Ireland, including completely unrestricted access to abortion during the first three months of pregnancy.
"Women’s lives are precious, to be loved, valued and protected. Their babies’ lives are precious, to be loved, valued and protected. Both lives deserve protection from the tragedy and irreversible decision of abortion.
"To be against abortion is not simply “a Catholic thing”. The innate dignity of every human life is a value for the whole of society - for people of all faiths and none. It is rooted in reason as well as in faith. To take away an innocent human life can never be simply a matter of personal choice.
"In recent months we have been reminded about the miracle of life in the womb - how your heart started beating from around week five, or your unique fingerprint began to form only ten to twelve weeks after conception.
"That little unborn child who moves her fingers or kicks around in the ultrasound scan is the same baby that will be born and grow further through infancy to adolescence to adulthood to old age - all that is needed for that life to grow, is time, nourishment, love, and a chance to survive.
"Say NO to repealing the Eighth Amendment and then do everything you can to ensure that our country will always provide the best possible care and support for all mothers and their unborn children."
The Archbishop's message comes as two polls released today show the yes campaign is ahead.
A Sunday Times Behaviour and Attitudes poll of 935 people between May 3 and 15 shows a five point increase for the yes vote in urban areas, with a surge of 12 points in Dublin to 67% and up 11 points to 54 % in the rest of Leinster.
Meanwhile a Sunday Business Post/Red C poll of 1,000 adults between May 10 and 16 also has the yes campaign ahead, up 3 points to 56%, against the no side, up one on 27%.