Micheál Martin described a letter of confession written in 1970 by the then Christian Brother Seán Drummond as "new information".
John Boland, representing survivors of Creagh Lane school, who were present in the Dáil gallery for the proceedings, said the Taoiseach should have been aware of the letter, which was revealed in an RTÉ Investigates documentary last year, featuring the survivors.
He said he was at a loss to know how Mr Martin was not aware.
Sinn Féin leader Mary Lou McDonald raised the plight of the men during Leader's Questions.
She pointed out that the men had been locked out of the limited State redress schemes for day schools because of the absence of evidence of a prior complaint against Drummond.
Ms McDonald said the confession letter itself constituted evidence of a prior complaint. It was concealed and never given to the survivors, she said.
The Taoiseach said: "I was not aware of the confession letter of Seán Drummond, which the Deputy outlined."
"I would like to get more background on that and to see what is the best way to take it from here in respect of the specific issue of an independent investigation into why that letter did not appear before now," he added.
The letter was highlighted by the survivors in the RTÉ Investigates 2025 documentary. A dozen Creagh Lane survivors spoke to the programme.
Drummond sexually abused all of them when he taught there in the late 1960s. They are now aged in their late 60s.
Survivors of the Creagh Lane abuse who spoke to RTÉ Investigates described lifelong effects of the abuse of them as children.
When some of their parents told the Christian Brothers of Drummond’s abuse in the 1960s, he was moved to CBS Sexton Street and Monastery nearby. He was later found guilty of abuse there too.
In 2004, one of his victims, ex-pupil David Phayer, went to gardaí and made a complaint, but there was no investigation.
He went back in 2007, and at that time, gardaí commenced a criminal investigation into abuse by Drummond.
In 2009, on the day his trial was due to begin in Limerick Circuit Court, Drummond, by then an ex-Christian Brother, pleaded guilty to indecent assault on 19 boys between the ages of seven and nine at Creagh Lane/Gerald Griffin Memorial Primary School, Bridge Street, Limerick between 1967 and 1968.
He was sentenced to two years in prison.
In 2013, Drummond again entered a late plea of guilty to two counts of indecently assaulting a pupil aged ten between August 1968 and August 1969 at CBS Sexton Street, Limerick.
He was sentenced to two years, but the sentence was suspended.
The Creagh Lane survivors took civil cases against the Christian Brothers.
Despite Drummond’s conviction, the abuse was denied. A decade of delay followed.
Drummond died in May 2021.
The congregation eventually settled with some of the men, but other cases remained in the courts.
But the letter, from 1970, cast new light on decades of denial and, victims argue, means the state cannot rely on a claim that there was no prior complaint of the abuse.
The letter shows that Drummond’s Christian Brother superiors, in Ireland and in Rome, knew of his abuse as far back as 1970 at least.
It is accompanied by two further letters, one from the leader of the Irish Christian Brothers, and a third from the Vicar General of the congregation in Rome.
Drummond’s superiors had transferred him out of Limerick to the Christian Brothers in James’s Street, Dublin after one incident.
In a two-page handwritten letter, written from the James's Street address on 24 February 1970, Drummond wrote to Superior General Br Arthur Loftus in Rome asking for a dispensation from his vows and admitting he sexually abused pupils.
He wrote: "I have interfered sexually with some of my pupils".
"Please accept my sincerest apologies for the harm I have done to the Congregation and pray for me that I may make up in some way the harm I have done to these boys," he said.
"I will always be thankful for all the good this congregation has done for me."
In a letter on 26 February 1970, which referenced orders for hand-crafted wooden Stations of the Cross and two statues ordered from Rome for Christian Brother monasteries in Ireland, the then Irish Christian Brother leader Br Timothy Gall Moynihan wrote to the Vicar General in Rome that Drummond "has been in trouble with boys and now would rather be allowed to leave than wait until summer".
He wrote that the Irish council leadership had unanimously decided that Drummond "… should be dispensed and let go …".
Within a week, Vicar General Br Michael Columba Normoyle granted the dispensation and Drummond was gone, freed from his vows.
Having left the Christian Brothers in March 1970, he subsequently married and had children, all now adults.
Survivors of the Creagh Lane abuse who spoke to RTÉ Investigates described a loss of education and opportunity and trauma to themselves and their families.
A number said they had suffered mental health issues, addiction and suicide ideation and one described several suicide attempts.
When they took civil cases in the courts, they said the legal tactics adopted by the congregation were hostile and designed to delay.
Speaking outside the Dáil session, John Boland said it was long past time for the Christian Brothers congregation to "stop mistreating people in the courts".
"They should release the papers they hold and relieve the pressure on people," he said.
He added that he believes that every Christian Brother who asks to leave the congregation has to apply for dispensation from their vows.
"There must be a roomful of such letters in the Christian Brothers' records.
"These were never produced in court and a lot of people passed away before any of this was revealed" he said.
"But the knowledge was there."