The X Factor Australia judge also shared a sweet snapshot of himself hugging the teen actress with the caption, "A massive Happy Birthday to the coolest TEENAGER. Love you so much. So proud of ya Moo."
Ronan, 38, and his girlfriend Storm Uechtritz prepared pink cupcakes for the special occasion, using edible disco glitter to decorate heart-shaped cutouts.
Storm Uechtritz: "This Hot Pink Disco glitter is fab! #sparklecupcakes#edible#pretty"
The Australian TV producer, 32, shared a picture on Instagram of their efforts, writing, "Been baking love heart cupcakes since 5:30am #FirstAttemptEver."
Ronan and Storm first met on the set of The X FactorAustralia in 2010, and confirmed they were dating in 2012. They are currently based in London to be closer to the singer's children from his 13-year marriage to Yvonne Connolly.
Marie 'Missy' Keating — named after Ronan's late mother — is their second oldest child, alongside big brother Jack, 14, and eight-year-old Ali.
Last year, Missy launched her film career, appearing in The Sea and Dark Touch.