The 'What About Us' singer announced her exciting news on Monday morning (6 October), saying that she was thrilled to be joining Westlife's Kian Egan, singer Rachel Stevens and Irish musician Niall Breslin as a coach on The Voice of Ireland.
Una, 32, tweeted: "Super excited to announce that I'll be joining the judging panel for the next season. I'm on the search to find Ireland's next international superstar...can't wait to get started."
In a statement, Irish-born Una added: "I'm delighted to be a part of the fantastic team behind the Voice of Ireland, I love the show and I can't wait to sit in that chair.
"I know the talent that Ireland has to offer so that button is going to see a lot of action. I'm excited to be working with Bressie, Kian and Rachel but it will be me who finds this year's winner!"
Una and S Club 7 star Rachel are replacing former coaches Jamelia and The Cranberries' Dolores O'Riordan.
Rachel said: "I'm a big fan of The Voice so I can't wait to join the team of coaches on the Voice of Ireland this year. I'm sure it will be great fun, but most of all I'm looking forward to listening to the great vocal talent that Ireland has to offer each week and to securing a team of people who have excited me enough to make me push that button for them."
Series four of The Voice of Ireland will start in January 2015.
Una's new job comes shortly after The Saturdays wrapped their nationwide 'Greatest Hits Tour' in support of their Greatest Hits album.
She's also pregnant with her second child with rugby player husband Ben Foden.