More than 35,000 children were placed in church-run institutions
A Irish theologian has said the "dregs of society" ran Catholic institutions where children were abused by what he called "monsters."
However, Fr Vincent Twomey also praised many other members of religious orders for caring for vulnerable people.
He was speaking on BBC Radio Ulster's Sunday Sequence.
Father Twomey is a former doctoral student of Pope Benedict's and meets him annually.
Last month the Ryan report on child abuse exposed the scale of abuse in institutions run by orders of monks and nuns.
The Christian Brothers and Sisters of Mercy were among religious orders criticised for their conduct.
Acknowledging that priests, brothers and nuns in authority had known about "the reign of terror" in their institutions, Fr Twomey asked how it was possible for religious people who were devoted to Christ and the care of children to turn out to be "monsters".
He said many of them came from large and extremely poor families and their parents had pressurised them to enter congregations because of the social status and life-long security they offered.
Fr Twomey said many did not have real vocations, and were frustrated sexually because celibacy was not their choice.
He said what we were looking at were "the dregs of society in a certain sense".
Meanwhile Catholic Primate Cardinal Sean Brady and Archbishop of Dublin Diarmuid Martin will on Monday deliver a report on the Pope's response to the Ryan report to Irish bishops.
The two men met Pope Benedict on Friday in the Vatican.
Later this week the bishops are expected to issue a statement setting out their responses and those of the Pope to the Ryan report.