Hannigan has a history of theatre performance, having auditioned for musicals as a teenager but remaining largely unsuccessful except for being cast as Second Urchin in her school's production of My Fair Lady.She went on to start her own theatre group called "The Cowardly Lion Theatre Group".Hannigan appeared in a Jessamyn Fiore play titled The Mysterious World of Birds in September 2003; the play received performances in Dublin and Galway. As part of an advertisement for Oxfam's Make Trade Fair campaign she allowed herself to be drenched in melted chocolate.As part of the same campaign, she made major contributions to the Irish musical collective The Cake Sale,with lead vocals on the tracks "Some Surprise" (a duet with Gary Lightbody of Snow Patrol with whom she has been romantically linked) and "Last Leaf". She also contributed to the 2009 charity album, Sparks n' Mind, released in aid of Aware.Hannigan has also spoken of her intention to run the Dublin Marathon one day.