Theresa Hiney Tinggal is from Dublin and now lives in Bournemouth in the south of England.
Ms Tinggal discovered 16 years ago, when she was 48, that her parents were not her birth parents.
She spoke to RTÉ Morning Ireland reporter Jackie Fox. (Read full transcript below)
Theresa: "My uncle told me because I had a lot of issues with my mother. And I said I was going to go to Ireland to see her. And he said: 'Well, I don't think you're going to like what you hear.' And I said: 'Why?' And he said: 'Because she's not your mother.' I said: 'Of course she's my mother.' He said: 'I really don't know how they did it, Theresa, but I went home from England,' because he lived in the north of England, and he said 'you were a baby there and your father said to me - 'yeah, we got Theresa but not in the legal way'.'
"And after that, there was no questions asked. And people didn't ask questions in those days. I mean all of my cousins knew, and they just accepted me as Jimmy's child basically.
"I just could not believe it. And then I said: 'Well, what about my father?' And he said he's not my father either. And I said: 'But my birth certificate says I'm Theresa Hiney.' He said: 'You're going to have to ask your mom about that'."
Reporter: What did your mother tell you?
Theresa: "She told me that my father met a man in Dublin one day and he had a child with him. He said to my father:
'Yeah, we got this little girl, 'but not in the legal way'.
And my father said: 'We'd really like to have some more children but we can't.' And he said: 'Ok, I'll pass your details on if you're OK about it.' And my father said: 'Yeah, fine.'
"They lived in Cabra at the time. My mother said a chauffeur-driven car came to our house in Cabra which was very usual at the time. And this lady got out and she approached my mother's house and she told my mother that she knew of a young girl that was going to have a baby and it was going to be born in June. My mother said yes, she'd be very interested – it didn't matter if it was a girl or boy.
"And then, just before my birth, they got a telegram from the doctor saying that I'd been born. That was on the 9th of June. And then two days later, my adoptive mother went to the house. My mother picked me up. I was two days old. And the nurse said to her: 'Go out the side door because Brigid might be looking out the window.' And the nurse and my mother took me to the church in Donnycarney and had me baptised as Theresa Hiney."
Reporter: What was going through your mind when your mother was telling you this?
Theresa: "Oh god, I was just floored. Even now, it's 16 years later and sometimes I think: 'My god, did that really happen to me?’ And the more I searched, the more I realised I wasn't the only one.
"I wrote to every newspaper – almost every newspaper. I was interviewed several times. And each time I really thought: 'God, you know, if my mother's alive, she's got to see my appeal in the paper.' And I just lived with hope the whole time."
Reporter: Did you ever find your birth mother?
Theresa: "Well thankfully, through DNA, I found my family last year in Tipperary. Unfortunately, my mother died eight years ago. You know, and I've been searching all these years and I often wondered did she ever see anything in the newspapers. Because I know one of them was the Tipperary Star that I sent an appeal to. However, I've got lots of information about her. And I've got photographs of her. I've been to her grave. I've got cousins and they've been fantastic."
Reporter: Did they know what happened?
Theresa: "No, they didn't know. No. For a woman in the '50s, it was a big shame. But she was actually 30 [years old] when she had me. And she did go on to get married. And there's a lot of people out there that don't know they're adopted, never mind illegally adopted. People contact me and unfortunately they've literally got no information at all. Nothing. And we've never got any help from the government. Nobody has helped. Nobody stepped up and said: 'OK, we'll do a search.' Nothing."
Reporter: How do you feel about the revelations from the child and family agency Tusla that 126 children who were adopted over two decades had their births incorrectly registered?
Theresa: "There's a lot more than that. That's just the tip of the iceberg.
"I think there should be an audit of all adoption agencies. I think there should be prosecutions where possible. I'd like to see justice for everybody.
"In all the years that I've spoken to adoptees and done all of this, I have never once heard any of them say they wanted retribution, they wanted redress, none of that. They just want justice. They just want an identity."