The duo, called GGMK, will try to impress with their take on Cee-Lo Green's 2010 hit "Forget You", which will air on TV in Britain on Saturday night, February 16.
Missy is feeling quietly confident because she has the full support of her family and a special connection to her famous dad.
"He was my age when he started," she says. "Having dad come in to see us play was great but it made me the most nervous I have ever been. I came off stage dying to know what he made of it. It turned out that he loved it."
The teenager is Ronan's daughter from his first marriage to Yvonne Connelly, who joined the hopeful for her "The Voice" audition.