The 33-year-old singer - whose 12-year union with former model Yvonne Keating was rocked earlier this year by revelations he had had an affair with a dancer - has used the sleeve notes of his latest album `Duet` to praise the "light" she brings to his life.
He wrote: "You are the most incredible, inspirational woman I have ever met.
"For the mistakes I have made in my life, I am sorry. Guess you have to go there to come back. It was a dark place and you were the light to bring me back. I love you."
Although Yvonne initially threw the singer out of their marital home in Dublin, Ireland after discovering his relationship with backing dancer Francine Cornell, the pair are believed to have reconciled recently, prompting the `Life is a Rollercoaster` hitmaker`s public declaration of love.
Ronan - who has children Jack, 11, Marie, nine, and five-year-old Ali with Yvonne - recently revealed he and his family are considering permanently relocating to Australia, where he is currently a judge on `The X Factor`.
He said: We have never really talked about living in another country until we came down here Australia and loved it so much. I feel like an adopted son here, so I am very happy. I am thinking of coming out here next year, it makes sense for us all."