Margo was brought up in the small village of Kincasslagh, in The Rosses area of County Donegal, Ireland. She grew up in a Catholic family, with her parents Francis and Julia (née McGonagle) O'Donnell, and her siblings: John (the eldest), Kathleen, James; and the youngest Daniel, who is also a singer. Her father died of a heart attack when she was a young woman. Margo started performing country music at a very young age in 1964 with a local showband, The Keynotes. She recorded her first single in 1968, "Bonny Irish Boy/Dear God", which was a success as was her second single, "If I Could See the World Through the Eyes of a Child/Road By the River", released in 1969. Margo has been a successful singer for five decades and has sold more than 1,000,000 records to date, and performed with Johnny Cash, Loretta Lynn and Dolly Parton. She presented numerous TV shows for RTÉ in the 1970s and has collected many awards during her career. Her albums usually include covers of popular country music hits from the 1960s and 1970s. Her 2019 album "Old and New" contains original tribute songs to two of her greatest influences, Liz Anderson and Jean Shepard. She has recorded a number of songs written by Liz Anderson and also a number written by Dolly Parton and on her album Margo and Friends she duets with Parton on "God's Colouring Book".