The couple, who are parents to twin boys Rocco and Jay, aged six, welcomed a little girl named Gia on Wednesday morning.
Proud daddy Nicky, 35, took to Twitter to announce the happy news.
"Our beautiful Baby Girl Gia… Was born today at 1030am," he wrote. "She is absolutely beautiful, just like her mommy! We are thrilled, Nico, G, Rocco & Jay."
Nicky's former bandmate Shane Filan was among the first to send his best wishes. "Huge congrats to @GginaAhernByrne & @nickybyrneoffic on the arrival of their little princess Gia," he wrote.
Jodi Albert, who is married to Kian Egan, said, "@nicybyrneoffic amazing!!!! Congratulations guys!!!! Big kisses to your beautiful little girl!!!!"
New mum Georgina — the daughter of former Taoiseach Bertie Ahern — has become a vocal advocate for pre and post-natal healthcare in recent years, following the early arrival of the couple's twins.
She contributed to a book published for the Irish Premature Babies charity called Tiny Footprints, in which she spoke candidly about the stress put on her and Nicky.
"It had not been the most comfortable of times, being pregnant with twins on a tour bus, attending concerts, etc," she revealed.
"I was happy and relieved when we got back home to Dublin, thinking we had at least a couple of weeks to get ready.
"Soon after we settled in back home we went to visit Nicky's parents; I remember it was a Wednesday. While we were there, they presented us with two adorable carrycots.
"Looking back on that day, I can clearly remember going to bed that night, thinking perhaps something was about to begin a wee bit too early.
"That next morning, we dashed into the NMH in Dublin, where I discovered I was going into premature labour.
"In the early hours of Friday, 20 April, the boys were delivered by emergency Caesarean section, eight weeks prematurely. And so began our journey into the world of precious premature babies.
"We're very different as individuals, but we're very close as well. He's my closest, closest friend and companion, and what I love about him is that he’s everything I'm not.
"He's so laidback and he makes me feel relaxed in his company, and he's very funny and charming, which I love. I'd probably be more reserved and shyer and a worrier – I’ll say something and then I'll think, "Is that terrible?"