The activities of Ireland's most infamous paedophile priest sparked concerns years before he was ordained, an inquiry has heard.
There was suspicion that Fr Brendan Smyth, who admitted sexually assaulting hundreds of children, had abused a boy while training in Rome during the late 1940s.
The revelations were made to the Historical Institutional Abuse (HIA) inquiry, which is examining whether systemic failings allowed Smyth - a member of the Norbertine order - to perpetrate the abuse over more than four decades.
Jospeh Aiken, counsel for the inquiry, said: "The Norbertine order believes that knowledge of Brendan Smyth's activities exists prior to his ordination yet he was ordained as a priest in any event.
"A complaint had been made about Smyth when he was a student in Rome in the 1940s. He was accused of abusing a child in the vicinity of the college."
Retired judge Sir Anthony Hart is leading the HIA probe, one of the UK's largest inquiries into physical, sexual and emotional harm to children at homes run by the church, state and voluntary organisations.
He was told that advice from a senior cleric in Rome, not to ordain Smyth, had been ignored.
Smyth's direct superiors at Kilnacrott Abbey in Co Cavan and the Belgian-based Abbey which sponsored his study at the Vatican felt it would be a "shame" if the first student they sent to Rome failed.
A letter shown to the inquiry also revealed that they did not want the Abbott General - the Norbertine's most senior figure in Rome - "interfering" in their business.
Smyth's ordination went ahead in 1951.
But, shortly afterwards, a senior priest from Tongerlo in Belgium sent a letter in which he said the Abbott General's opinion may have been right.
"My letter is hard," he wrote. "I hope my fear is exaggerated."
Smyth, who was at the centre of one of the first clerical child sex abuse scandals to rock the Catholic Church, was convicted of 117 indecent assaults on children in Northern Ireland and the Republic of Ireland from the 1960s to the 1990s.
There were also reports he abused children in Scotland, Wales and the United States of America.
The inquiry heard how the prolific child molester told a doctor in 1994 - the year he was jailed - that the true number of victims could run into the hundreds.
Mr Aiken added: "In his own words, which have not been heard in public before, Brendan Smyth said to a treating doctor, 'over the years of religious life it could be that I have sexually abused between 50 and 100 children. That number could even be doubled or perhaps even more'."
It was revealed Smyth had a preference for children aged between 10 and 14 years old because he felt they would not inadvertently speak about the abuse.
He frequented Catholic residential homes including two institutions run by the Sisters of Nazareth in Belfast and the De La Salle Boys' home in Kircubbin, Co Down, where he groomed victims with sweets and trips away.
His deviant behaviour was known among several of his superiors, who recommended psychiatric treatment.
Throughout his lengthy religious career, Smyth received psychiatric treatment including electric shock therapy in Northern Ireland, the Republic of Ireland and in England as far back as the 1960s.
Penalties such as the withdrawal of confession faculties and having to get permission to use a car were also imposed.
But, when the the cleric was transferred between parishes, dioceses and even countries, he was given a clean slate because no details about his deviant behaviour were passed on.
Mr Aiken said: "What is now known is that his abuse of children continued as well as his treatment."
By 1978 he had not been reported to the police - and he was not arrested until the 1990s.
Mr Aiken later said: "In all of this... there still remains not one discussion about any child, their parents or reporting Smyth to the authorities.
"It won't take long to count up the number of instances by June 1978, to see just how many opportunities to take action were missed."
In a statement, Fr William Fitzgerald, from the Norbertine order who has been assisting the inquiry, said Smyth should never have been ordained into the priesthood.
He said: "I accept that Brendan Smyth was not a fit person to have access to children at any time or under any circumstance.
"I am ashamed by the failure as a community to hear these warnings and act accordingly.
"The shame of our failings is immense."
Fr Fitzgerald is due to give oral evidence on Wednesday.
The HIA inquiry is being heard at Banbridge Courthouse, Co Down.
Smyth's abuse has already been described by a number of witnesses who have previously given evidence.
This week the panel is concentrating on an examination of what opportunities there were to prevent Smyth carrying out the abuse of children and whether any action, or inaction, amounted to systemic failings.
Former Primate of Ireland Cardinal Brady, who was part of a much criticised church investigation in 1975, is among those due to give evidence later in the week.
In one of three written statements, the Cardinal said: "Sadly at that time there was a culture within the church of secrecy and silence and it was felt that matters could be dealt with within the church structures.
"There was not a proper understanding of the devastating consequences of child abuse. Many of the bishops believed that psychiatric treatment of the individual perpetrator was an adequate response. The full horror and long-lasting impact of such criminal behaviour has now been grasped."
Smyth died aged 70 from a heart attack in prison in the Republic of Ireland in August 1997.
Mr Aiken said his legacy was a "piece of history truly deserving of the title a public scandal".
The inquiry was formally established in January 2013 by the Northern Ireland Executive.