The Tipperary girl, who took part in a Christmas edition of School Around The Corner recently, will be appearing on a special episode of Who Wants To Be A Millionaire which airs at 8pm tomorrow.
But at least the well-known singer won't be facing the daunting challenge alone -- she'll be joined by her rugby star hubby Ben Foden.
The 31-year-old said she relished the chance to appear on the ITV1 programme to raise some money for a good cause.
"Ben and I are huge fans of the show and have watched it for years," she said. "When we were asked if we wanted to play for our chosen charities Marie Curie and Niamh's Next Step, we jumped at the opportunity.
"We can't wait to take part and hopefully raise lots of money for these great causes."
Other celebrities who have agreed to take part include wildlife expert Bear Grylls, singer Michael Ball and Olympic medallist Ellie Simmonds.
Una has just spent a relaxing Christmas at home with her family in Thurles after a hectic few months for her and her chart-topping band working in the States as she and her band members try to crack America. Their endeavours were filmed for a show for E! which is due to air in January.
Called Chasing The Saturdays, it follows all the antics of Una and fellow singers Mollie King, Rochelle Humes, Frankie Sandford and Vanessa White in LA.