Rocco and Jay, who were born in 2007, marked the occasion with their parents at the Westbury hotel off Grafton Street in Dublin’s city centre.
Taking to social media, proud dad Nicky shared a throwback picture of himself and his two boys when they were toddlers.
The picture showed Nicky holding his boys hands as they walked down steps wearing Dublin GAA jerseys with their names emblazoned on the back.
Another snap shared by the Westlife singer showed the boys with their dad as teenagers posing at Old Trafford, home of their beloved soccer team Manchester United.
“Our boys turn 15 today,” he captioned the post adding: “Becoming fine young men before our very eyes.
“Happy birthday lads, love always.”
In another post, Nicky shared a gorgeous photograph posing with his wife of 19 years Georgina Ahern.
“Celebrating Rocco n Jay’s 15th,” he wrote.
The image shows Nicky wearing a blue shirt with blue jeans and light coloured grey boots as he poses alongside Georgina.
Georgina is wearing a white dress with long sleeves rolled up to her elbows and a pair of high heeled gold sandals.
The couple are also parents to 8-year-old Gia, born in 2013.
Nicky is set to return to the stage with Westlife later this year.
The megastars are set to play the Aviva Stadium, Dublin on July 8th, and Saturday, July 9th.
Mark, Nicky, Shane and Kian will also Páirc Uí Chaoimh in Cork on August 12th and 13th.
They’re also due to play London’s Wembley Stadium on August 6th, as well as a whole host of venues across the UK.
Fans will have the opportunity to see the famous foursome perform their biggest and greatest hits like ‘Swear it Again’, ‘Flying Without Wings’, and ‘World of Our Own’, as well as tracks from their upcoming album ‘Wild Dreams’.