"I was truly bowled over when I first saw Once," Keating said. "The music and storyline are so incredibly moving and combined with John Tiffany's direction, I believe it's a modern-day masterpiece."
The role of Guy was originated in the West End by Declan Bennett, currently on our screens as Charlie Cotton in EastEnders.
Doctor Who star Arthur Darvill replaced Bennett for a limited run, and David Hunter, who has been a long term understudy of the role, recently took the role on full time opposite new Girl Jill Winternitz - you can see our interview with them here.
Keating will take over from Hunter for a limited four month run from 17 November through to 21 March.
He added: "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."
Keating, whose solo hits include "When You Say Nothing At All" and "Life is a Rollercoaster", teased his twitter followers earlier today, hinting at 'incredible news':
Got some incredible News for everyone. So excited. Will tell you all tomorrow. Or maybe even sooner.— Ronan Keating (@ronanofficial) June 5, 2014
It's all going to be happening at the end of the year !!!!— Ronan Keating (@ronanofficial) June 5, 2014
And he confirmed the news this evening, tweeting:
So the News is out. Incredibly excited. Lots of work to do. But really can't wait. @OnceMusicalLDN— Ronan Keating (@ronanofficial) June 5, 2014