At least 126 births were illegally and incorrectly registered over a 23-year period, the Irish government has announced.
An independent review will be launched into the cases between 1946 and 1969 in which those affected may not know their parents were not their birth parents, the minister for child and youth affairs said.
Katherine Zappone said each individual would be contacted by Tusla, the Child and Family Agency, and that she was "truly sorry" for what happened.
Dr Zappone said the information had come to light after the index cards on 126 files - taken from 13,500 records from former adoption society St Patrick's Guild - were found to contain the words "adopted from birth".
She added that those responsible may have been motivated by a belief that "this was best for the child", and suggested more people could be affected.
Speaking at a press conference in Dublin, Ms Zappone said: "I am keenly aware that many of those affected have no idea that the people they regarded as their parents were not in fact their birth parents.
"I have tried to put myself in the shoes of those that are going to be given this news - quite frankly it is impossible. Our identity goes to the heart of who we are.
"For people who are middle age and older, to be told at this stage in their lives that their parents are not their birth parents and their births were deliberately falsely registered will be nothing short of traumatic for them and those around them.
Addressing who would be affected, the minister said: "If you or your child has an adoption order you are not affected. If you were born before 1946 and after 1969 you are not affected. If you were born between 1946 and 1969 and were placed by St Patrick's Guild and do not have an adoption order, you may be affected.
"As minister responsible for both Tusla and the Adoption Authority, let me state that we are not out to destroy, upset or split families.
"It is hard to estimate the true extent of incorrect registrations of birth because this practice was concealed."
Concluding her statement, Dr Zappone said: "To those who lost their true identity and to the birth mothers who placed their children in good faith, thinking they would be legally adopted, I am truly sorry this has happened."