"Deborah looks fantastic and lost four stones -- I could kill her," Mary laughed.
"She's a big gym person so I'm going to join and start to go swimming.
"I'm on a lovely eating regime with my daughter. I have an underactive thyroid so the weight is coming off very slowly. But it's good and I feel good," added the former Tesco worker.
Mary (53) says that she would like to slim down for the good of her career.
"I hope to be doing more work next year, so I need to be healthier."
Mary, who also battles with arthritis, does not want to slim down too much.
"It's for myself as well; I don't want to be skinny, but I'd like to be a few stones down and feel that I can move around and do different things."
The singer is back in her native Ballyfermot for Christmas and says she couldn't be happier as looks forward to a traditional knees-up with her family and friends.
"I'm spending a quiet Christmas with my family -- cooking the turkey at home and having a nice dinner with Deborah.
"We'll have a hooley on Christmas night," added the 53-year-old Dubliner, who shot to fame on the UK X Factor in 2010.
While Mary is looking forward to what 2013 will bring, she is staying away from making resolutions.
"I don't make them any more because, every time I made them, I broke them the following day."
Mary was speaking at the Heinz Celebrity Ward Walk in Crumlin's Children Hospital, where a number of Irish celebs met and sang carols with some of the children.
She was joined by comedian Brendan O' Carroll and singer/ songwriter Bressie as she sang Silent Night.
Also at the traditional Christmas walk were Monaghan singer Ryan Sheridan, model Holly Carpenter and Dubliner Brian Ormond. The event was organised by Valerie Keating and her husband Gary, brother of Boyzone star Ronan.