The show, based on the film of the same name, is a love story between an Irish singer and Czech woman who meet on the streets of Dublin.
Keating, who joins the cast on November 17, said he was "truly bowled over" when he first saw the show.
He said: "The music and storyline are so incredibly moving and combined with John Tiffany's direction, I believe it's a modern-day masterpiece. It's unlike anything else I've ever seen.
"When I was asked about playing the lead male (who is a busker in Dublin) I was completely overwhelmed because I knew it would be perfect - this was my home town, my people, and the place where I began my own music career.
"But as exciting as that is, it's scary too. I've never performed in the West End and this is a huge challenge and commitment. I'm really looking forward to returning to London later this year and completely sinking my teeth into the role. I think there's going to be lots to learn and lots to gain from this experience and I just want to be the absolute best that I can be."
The West End production, which opened in April 2013 and won two Olivier awards this year, is at the Phoenix Theatre in Charing Cross Road.