The 33-year-old posted the picture to Instagram, which showed Aoife Belle, three, tenderly stroking her baby brother Tadhg’s face.
Saturdays singer Una also uploaded one of doting dad Ben, 29, carrying his son in a sling.
He’s just six weeks old, but little Tadhg is already very animated – as Una showed with a super cute montage at the weekend.
Ben and Una helped Aoife celebrate her third birthday on Mother’s Day with a party for her and all of her little friends.
The proud mum shared a snap of Aoife blowing the candles out on her Peppa Pig birthday cake, writing: “Aoife had the best birthday party with her little friends x”
Una and Ben recently revealed how excited Aoife was by the birth of her brother.
Speaking to Hello! magazine, Una said: “We’d explained to Aoife the night before that this was the day her little brother would arrive.
“She came in with this little teddy bear for him, went straight to his bedside cot and started singing Twinkle, Twinkle Little Star.”
Ben added: “It was the first time we’d heard her sing it all the way through so that tugged at the old heart strings.”