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McMahon is from Dublin. She was classically trained in voice and harp. She studied Italian and Drama Studies at Trinity College, Dublin. During a break from her studies, Katie joined the vocal group Anúna in 1991, and remained with the choir until 1996. She was a soloist on Anúna's first three CDs "ANÚNA" (1993), "Invocation" (1994) and "Omnis" (1995).
She is best known as the original lead soprano soloist with Anúna in the piece "Cloudsong". This is the opening section of Riverdance, first performed as part in the 1994 Eurovision Song Contest. Subsequently she went on to participate with Anúna in the offshoot stage performance "Riverdance the Show" until 1996, when she left the choir, remaining with the show until the late 1990s.
Her debut album, After the Morning, was released in North America. Since then she has released three more, Shine, Celtic Christmas, and St. Patrick's Day.
She is married to Ben Craig with whom she has two children. She currently lives in Minneapolis, Minnesota.