Katie Lucia Keegan, 19, was stunned as "best pal" Ryan revealed his secret feelings for her in the moving ballad No Name.
She told pals on Twitter: "This is sooooooo funny."
And last night Ryan said he wants to whisk her away on holiday and buy her a "sparkly ring".
The Sun told yesterday how songwriter Ryan, also 19, moved the audience to tears at his audition and sparked a guessing game over his sweetheart's identity when the footage was screened on Saturday.
Revelation ... Katie's Twitter post after hearing song
Even judges Simon Cowell and David Walliams wanted to know more, with Simon tweeting Ryan after the show.
Last night Ryan — who sang the words "I'm falling deeper in love" and "I try to talk but I can't" — said that he and Katie had since spoken twice over the phone.
He told The Sun: "She's loving it and is flattered. But I don't think she realises I'm in love with her yet."
The pair met at Portmarnock Community College in Dublin six years ago and became mates. But Ryan realised he had fallen in love when he took Katie to their school prom last September. He said: "When I saw her in that dress, I thought she looked beautiful.
Secret ... Ryan O'Shaughnessy performing on Britain's Got Talent
"She makes me nervous. I don't want to rush in. She's my mate, she's a nice girl and has a beautiful smile.
"If I win I'll spend my £500,000 on a holiday for just the two of us, and maybe a sparkly ring if she's lucky."
Pals said Katie is "overwhelmed". One, Dan Malone, added: "She's the girl they all fancied at school."
Ryan's future on BGT is under threat because earlier this year he appeared on the Irish version of The Voice and signed a contract with Universial Music.
He was not able to sing his own songs on that show — and is now begging bosses to free him. He hopes public support will help win them over.
He said: "On Britain's Got Talent I came across much better. If I could sing in the finals it would mean the world."