O’Shaughnessy wowed Simon Cowell and the other talent show judges with his song No Name, a self-penned love song.
The judges were blown away by the performance and he was immediately tipped to win the entire competition.
However the Dublin native soon became embroiled in controversy as it was revealed he had already been a contestant on The Voice of Ireland, but was eliminated after the first week of the live shows.
His dream for success on BGT was shattered on Saturday when he was told by Cowell he would not be going through to this week’s live semi-finals, as he was already contracted to Universal and the Irish show.
Viewers saw Cowell telling the 19-year-old student: “You are on another show which you’re signed up to.”
O’Shaughnessy explained to the BGT boss: “I wasn’t allowed to express my own music on the other show.”
Now the former Voice of Ireland contestant has managed to get himself out of contractual obligations, and viewers will see him compete tomorrow night in the last semi-final of BGT.
It is has since been revealed the song that wowed the BGT judges was written about his friend Katie Lucia Keegan.
Cowell reacted to his performance by saying: "Have you played her that song? So that's the first time she's ever gonna hear it? Wow.
"Well, if that doesn't work, nothing will. Are you sure you don't want to give me the name? But she's gonna know when she watches this, I presume? I think you've got a brilliant sense of melody, I think your lyrics are superb. And I love your voice."
Alesha Dixon commented: "Ryan, that was really sweet and sincere. I really enjoyed it. And I felt it. It was really lovely. Well done."
And David Walliams added his praise: "I can't believe she won't go out with you. That's one of the most beautiful songs I've ever heard, sung incredibly well. That was absolutely astonishing. Well done."
However romance is not on the cards for the pair, as Keegan is believed to be seeing someone else.
Keegan was aware that O’Shaughnessy had gone for an audition for the ITV talent show, however was unaware of the song he was singing, or the fact it was about her.
O'Shaughnessy who also appeared in Fair City had always said he would prefer success on BGT than the Voice of Ireland: "I entered BGT last year but soon afterwards my mum entered me in The Voice.
"I didn't know she was going to do it, but we thought, 'what harm can it do'. We had no idea I'd get so far.
"I've always wanted to be on BGT so it would be a real blow if I had to leave. It's the better stage."
O’Shaughnessy will compete in the final semi-final tomorrow evening, with the live final on Saturday night.