The singer - who reunited with the 90s group in 2007 after a seven-year break - is more conscious of how he looks now and has embarked on a strict exercise and diet regime.
The father-of-three said: "Being in a boy band, I'm very aware of keeping myself in shape, and it makes you a little bit vain! I work out with a personal trainer every day in the gym, and I don't eat carbs. I eat plenty of protein like beef, chicken and veg, and absolutely no chips, rice or potatoes. I'm in the best shape of my life."
Ronan also revealed he and his wife of 11 years have no plans to add to their family as they are quite happy as they are.
He added to Britain's Closer magazine: "Becoming a dad is the best thing I've ever done. But it's hard work at times, and Yvonne and I have decided we're happy with the three kids we've have.
"I miss the kids terribly when I'm away touring, it's the toughest thing about my job. Not being there to put them to bed or collect them from school.
"But while I'm not planning on having more, I'd be over the moon if we got a surprise addition to the family."
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