The dedicated new mum decided to forgo any birthday treats and shunned cake to resolutely stuck to her diet plan.
And last night, viewers of the hit RTE weight loss show saw how Deirdre's efforts are paying off. She lost five pounds, even though her target for the week was just two pounds.
"There was no cake this year," laughed Deirdre, who is a busy mum to Jack (2) and Cian (four months).
Her garda husband Will and the boys put a candle in the apple and sang "Happy Birthday" to her, when she turned 33 last Friday and her mum also made her sugar-free jelly.
Deirdre, who lives in Co Cork, was 14 stone and six pounds, and in the first week of Operation Transformation lost seven pounds. Her total weight loss so far is 12 pounds.
The glamourous stay-at-home mum told the Herald that she wasn't expecting to lose that much weight this week: "I am over the moon. I had worked hard and I did stick to the plan."
She said that she wasn't tempted to break with her plan on her birthday last Friday: "I am going to do it to the best of my ability."
Deirdre said that she has been getting massive feedback from mums. "It is amazing to have the support of all these people."
She said that she may not lose five pounds every week, but as long as she consistently loses weight she will be happy. Last night's programme also saw the leaders being put through their paces in the Curragh as part of the army challenge.
Leader Greg Starr (36) also did well this week, losing four pounds. His starting weight was 17 stone, 11 and a half pounds. He has lost 15 and a half pounds in the first two weeks of the sixth series.