Jon Kopaloff/FilmMagic
The baby, who weighed 7 lb., 10 oz. and was 20.5 inches long, is the third child for Maguire, 38, and her actor husband, Gareth Maguire, 33. The couple also have twin girls, Eva Ruth and Kathleen Emilie, who were born in April 2004.
"Martie is doing very well and Gareth and the family are over the moon," says a statement on the band's Web site.
In January, Maguire announced her pregnancy to PEOPLE – and revealed that she knew the baby's sex. "I'm very excited," she said at the time, joking, "Carrying one instead of two will be much easier."
