Robert Doherty, 72, was found guilty of six counts of indecent assault by a jury at Galway Circuit Criminal Court this morning.
He had denied the offences, which took place on dates between August 1965 and April 1967.
Doherty, originally from the Falls Road in Belfast and with an address at Glenwood Estate, Dundalk in Co Louth, was recruited to the Christian Brothers at the age of 15.
In 1965, he began teaching at Letterfrack Industrial School at the age of 24.
He initially had only a teaching function but a year after his arrival, he was put in charge of the boys detained there.
He worked in Letterfrack until 1968 and left the Christian Brothers order in 1976.
The retired primary school teacher had denied six charges of indecently assaulting a young boy on dates between August 1965 and April 1967 in his bedroom beside a dormitory in Letterfrack Industrial School.
The victim, who is now 59, was sentenced to four years' detention in Letterfrack at the age of nine for taking a bike for a few hours before returning it to a garda station.
He said the facility was ruled by fear and he could only remember one Brother who did not abuse him during his time there.
The man said the abuse happened on a regular basis over a considerable period of time.
Doherty had denied all the allegations, stating that no boy had ever entered his room during his time at Letterfrack Industrial School.
After the jury resumed deliberations this morning, they returned majority guilty verdicts on all six counts.
The 72-year-old has been remanded on bail until 15 May when medical reports and a victim impact report will be presented to the court.
Speaking after the verdict, the victim thanked gardaí for the support they had given him since he first reported the abuse in 1999.
He said he had dreaded the legal process many times, but was glad that he had never given in and that he had never given up hope.