After leaving secondary school, Brennan spends a few years at the Royal Irish Academy of Music in Dublin studying the harp, the piano, and singing. She has also taught music at Holy Cross College in Falcarragh, County Donegal.
In 1970 Brennan joins her two brothers, Pól and Ciarán, and their mother’s twin brothers, Noel and Pádraig Ó Dúgáin, and form the band Clannad. They are identified and introduced to television by Tony MacMahon. After enjoying a decade of being among the world’s foremost Irish musical groups, Clannad graduates to chart success in 1982 with the album Magical Ring. Brennan is at the forefront of the group’s success and her voice suddenly becomes synonymous with Celtic music and Irish music at the time. Brennan records 17 albums with Clannad and wins a Grammy Award, a BAFTA, and an Ivor Novello award with the quintet. Her sister, Eithne Ní Bhraonáin, who spends some time with Clannad, continues to pursue a very successful solo career under the name Enya.
Brennan releases her first solo album in 1992, Máire, on Atlantic Records. Misty Eyed Adventures on BGM follows three years later. In 1998, Brennan signs with Word Records and releases Perfect Time, and Whisper to the Wild Water a year later. The album is nominated for the Grammy Award for Best New Age Album in 2001.
Brennan’s autobiography, The Other Side of the Rainbow, is published in 2000 and she performs her song Perfect Time live at World Youth Day in Rome in front of crowds of pilgrims and Pope John Paul II. There are 2.1 million people present, making it the largest crowd ever gathered in the Northern Hemisphere.
In film, Brennan is a featured vocalist on King Arthur (2004), co-writing the title theme Tell Me Now (What You See) with Hans Zimmer. She also writes an additional music score for To End All Wars (2001). In 1995, she duets with Shane MacGowan on You’re the One for the movie Circle of Friends. Brennan has collaborated with many other musicians, including Chicane, Alan Parsons, Bono, Robert Plant, Van Morrison, Michael McDonald from the Doobie Brothers, Bruce Hornsby, Joe Elliott, The Chieftains, Paul Young, Paul Brady, Michael Crawford, Joe Jackson, and Ronan Keating.
In total Brennan has recorded 25 albums, and has sold 20 million records. Brennan and Clannad are credited with the creation of contemporary Celtic music and are held in high esteem for their vast contribution to bringing new life to old Irish songs. They have been compared to Seán Ó Riada, in that they brought the Irish language into popular culture through their music. One critic said, “Clannad’s music offers a terrific fusion between traditional and modern influences.” U2 front man Bono says of her voice, “I think Máire has one of the greatest voices the human ear has ever experienced.”