The church hierarchy said the deliberate decision to deprive a human being of life is always morally wrong.
"The heads of the Protection of Life during Pregnancy Bill 2013 published by the Government would, if approved, make the direct and intentional killing of unborn children lawful in Ireland," the bishops said in a statement.
"The bill as outlined represents a dramatic and morally unacceptable change to Irish law and is unnecessary to ensure that women receive the life-saving treatment they need during pregnancy."
The Bill was published late on Tuesday night after intense discussions between the coalition parties Fine Gael and Labour. Taoiseach Enda Kenny held talks with his own parliamentary party on Wednesday amid concerns of a backbench revolt.
Fianna Fail has not revealed whether it will tell its TDs to vote for or against the legislation despite the parliamentary party meeting for four-and-a-half hours on Thursday evening.
The proposals, if enacted, will legislate for the 1992 X case judgment from Ireland's Supreme Court which found abortion is legal if there is a real and substantial risk to the life of the mother, including emergency medical cases, non-emergency medical cases and the threat of suicide.
The case was taken by a 14-year-old rape victim who became pregnant and was refused permission to travel for an abortion.
As well as that judgment, the loosening of rules is intended to meet requirements from a European court decision that found a woman in remission from cancer should not have been forced to travel overseas for an abortion.
The bishops said their statement was a preliminary response to hugely contentious reforms and they also raised concerns that an abortion could be carried out in a Catholic hospital.
Cardinal Sean Brady said the bishops have not discussed whether politicians who advocate support for the legislation should be barred from communion, as has happened in some US dioceses.
"There would be a great reluctance to politicise the
Eucharist," he told RTE Radio. "I say that they (politicians) have an
obligation to oppose the laws that are attacking something so
fundamental as the right to life and they would have to follow their own
conscience."