The 'Up' singer is poised to marry her England rugby ace fiance Ben Foden at a secret venue near Thurles, Ireland.
Una's bandmates - Mollie King, Frankie Sandford, Vanessa White and Rochelle Wiseman - will be bridesmaids, while other guests will include boy band JLS and Ben's team mate Mike Tindall and his wife Zara Phillips, who is 14th in line to the British throne.
Una, 30, gave birth to daughter Aoife Belle in March, and said last week: ''I want a nice, laid-back, fun day.''
Rochelle - who is marrying JLS star Marvin Humes - and Una had a joint hen party in May with their bandmates, partying at London's Rose Club.
Una has also said she has been inspired since Aoife's birth to pen a track especially for her.
She said: ''And she seems to like it when I sing to her. I'm writing her a song called 'Aoife Belle'. Having her has really inspired me to get back into writing music and playing guitar.''
The marriage is taking place just after Ben, 26, returns from England's rugby tour of South Africa.