He took secret singing lessons and eventually entered the Irish equivalent of "American Idol," a show called "Go for It."
He won, and went on to complete medical school. But he couldn't stop thinking about music. The other doctors took up a collection to help send him to the Royal Opera Academy in England.
He was 33. "It was late to start, there is no question," he said.
But he says he never thought he would fail. "That's not in my dictionary. That word is not there," he said.
He soon became part of the platimum-selling Irish Tenors, and recently went solo. This month, his CD, "Ronan," was released.
He's even become America's singer without being American. He's now a regular during the seventh-inning stretch at Yankee stadium, and sports a New York Yankees Championship ring.
"I've become a New Yorker, I've become an American actually. I absolutely love this country," he said.
Tynan says when he gets ready for concerts, he often thinks about his parents -- and what it would be like for them to see him now.
His father died seven years ago, before his enormous success as a singer. His mother has Alzheimer's and no longer recognizes him.
Tynan says time has helped him see her in a new light. He's grateful she never gave up. "I thought of all the wonderful things she had given me," he said. "I don't think I would be who I am today without her."
He has written a song for her. It's called "Passing Through."
"'Passing Through' was my thank you to her for everything, and all her dreams," he said. "And it was, 'Thank you for the tough love, because you were right.' "
To find out more about Ronan Tynan, visit his Web site at http://www.ronantynan.net.