Now weighing 11 stone, she has set herself a target of getting under the 10-stone barrier.
And the slimming bug has now hit Mary too. "I'm losing the weight now as well because she's encouraging me, and we both feel a whole lot healthier," she told the Herald.
"Deborah gets up now and has her breakfast, and a light lunch and a dinner later on.
"She has cut out the junk and the snacks and the comfort eating. We both have now," Mary said.
"And it makes such a difference to her, especially because she can now buy clothes off the rail whereas before she lost the weight it was all tracksuits and stretchy things," she said.
Deborah goes to the gym four times a week and does around 90 minutes on the treadmill, and enjoys swimming as well.
"She's basically eating healthier and getting some excerise, and that's all it takes to make the difference," said Mary.
She said her X Factor experience in 2010 has made a big difference to both their lives, and helped boost their confidence.
"Deborah saw the difference in my confidence, and I think that helped her do what she needed to get her own confidence up -- and that was lose weight," Mary said.
"I've noticed a great difference in her. We're the best of friends as well as mother and daughter, and we've always supported each other," she added.
She said an underactive thyroid means her own weight is coming off very slowly, but she is determined to keep with Debbie's new healthy habits.
"It's good, and I feel good," added the former Tesco worker .
While Mary (53), who also battles with arthritis, does not want to slim down too much, she is looking forward to losing some post Christmas pounds.
"It's for myself as well -- I don't want to be skinny but I'd like to be a few stone down."
Mary and Deborah will be spending Christmas in their native Ballyfermot, Dublin, with a traditional celebration with family and friends.