But unrestricted abortion access "would make the regime in Ireland one of the most liberal in the world," he said. "Many people are astonished and outraged by this proposal."
"What is being proposed will create a shocking and blatant inequality. We will have a two-tier value system where we regard the life of some people as valued and welcome but another is not allowed even to be born," he said.
"We must clearly state what abortion is – it is the direct killing of an innocent human being. This is simply not right," said the bishop. "Children with disabilities or genetic disorders are disproportionately affected. In Britain 9 out 10 babies who have been diagnosed in the womb with Down Syndrome are aborted."
"A measure of a truly civilized society is how the most vulnerable and defenseless are treated. The unborn child in the womb is among the most vulnerable and defenseless of all," he said. Passage of the referendum would have "alarming and far-reaching consequences" for others who are vulnerable.
He praised Irish medical care for mothers and babies as "among the best in the world." Women in difficult situations "need and deserve the very best of medical care, support and love."
"Cherishing human life concerns us all," said Bishop Monahan. "The love of a mother for her baby in her womb or embraced in her arms, is the most powerful expression of love in our world. When the Bible seeks to convey how much God loves us, his people, it uses the image of a mother's love for her child."
The mother, father, wider family, and broader community "play their part in forming a circle of love and care to embrace the mother and her unborn child," the bishop said.
Efforts to repeal the Eighth Amendment have drawn international attention, including donations from George Soros' Open Society Foundations that ran afoul of Irish political finance rules.
Facebook recently announced it will ban foreign funding of ads related to the abortion referendum. The search giant Google went one step further, saying it would not run any foreign- or domestic-backed ads on the referendum.
John McGuirk, a spokesman for the Save the 8th campaign, welcomed Facebook's decision but he argued that Google's move against domestic advertising was driven by fears of pro-abortion rights groups that they would lose and therefore wanted to limit voter information.
Supporters of the Eighth Amendment said mainstream media is dominated by pro-repeal voices and online media is their only platform to speak directly to voters.
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The Pro Life Campaign, Save the 8th and the Iona Institute said in a joint May 9 statement: "It is very clear that the Government, much of the establishment media, and corporate Ireland have determined that anything that needs to be done to secure a 'Yes' vote must be done."