The announcement contradicts a 2011 campaign promise by Prime Minister Enda Kenny that his government would not introduce abortion in the predominantly Catholic country.
In a statement, Ireland's four top Catholic churchmen said everyone should be concerned by the announcement.
"If what is being proposed were to become law, the careful balance between the equal right to life of a mother and her unborn child in current law and medical practice in Ireland would be fundamentally changed. It would pave the way for the direct and intentional killing of unborn children. This can never be morally justified in any circumstances," said their statement.
It was signed by Cardinal Sean Brady of Armagh, Northern Ireland; Archbishop Diarmuid Martin of Dublin; Archbishop Dermot Clifford of Cashel; and Archbishop Michael Neary of Tuam.
They said human dignity depended on "our respect for the right to life of every person from the moment of conception to natural death. The right to life is the most fundamental of all rights. It is the very basis for every other right we enjoy as persons."
They also urged that, when the legislation is introduced, lawmakers be allowed to vote according to their consciences.
"All involved, especially public representatives, must consider the profound moral questions that arise in responding to today's announcement by the government. We encourage all to pray that our public representatives will be given the wisdom and courage to do what is right," they said.
In practice, abortion has been illegal in Ireland under 1861 legislation. However, a 1992 Supreme Court judgment -- known as the X case -- found that there is a constitutional right to abortion where there is a substantial risk to the life of the mother, including the risk of suicide, up to birth.
Successive governments have not acted on the issue. However, the European Court of Human Rights ruled in 2010 that Ireland must clarify when women can access abortion under the 1992 ruling.
After a Cabinet meeting Dec. 18, Reilly announced that the government would introduce legislation to allow abortion where there is a risk to the life of the mother, including the threat of suicide. He also confirmed that the government intends to decriminalize abortion in these circumstances.
Speaking after the Cabinet meeting, Reilly said the legislation would clarify "what is available by way of treatment to a woman when a pregnancy gives rise to a threat to a woman's life."
"We will also clarify what is legal for the professionals who must provide that care, while at all times taking full account of the equal right to life of the unborn child," he said.
However, Dr. Ruth Cullen, spokeswoman for Ireland's Pro-Life Campaign, said that "any legislation for the X case would blur the distinction between life-saving medical interventions in pregnancy and induced abortion, the sole aim of which is to intentionally end the life of the baby.
"Once it is conceded that some human lives may be directly targeted, there is no going back. Inevitably over time the grounds for abortion would be widened," she said.
The Irish parliament will debate the proposed legislation in early 2013. Several legislators from Kenny's Fine Gael party have indicated that they would oppose the legislation.
Pro-life campaigners are calling for a mass rally Jan. 19 to express opposition to legislation.