Donnacha Dennehy (born 17 August 1970) is an Irish composer and leader of the Crash Ensemble specializing in contemporary classical music.

According to musicologist Bob Gilmore, Dennehy's "high profile of his compositions internationally, together with his work as artistic director of Dublin’s Crash Ensemble, has distinguished him as one of the best-known voices of his generation of Irish composers".

His composition Grá Agus Bás, which was premiered in February 2007, incorporated music from the sean nós tradition and was a collaboration with the Irish vocalist Iarla Ó Lionáird.

[4] His first opera, The Last Hotel, an 80-minute chamber work with a libretto by Enda Walsh about a woman planning her suicide, received its premiere on 8 August 2015 in Edinburgh, followed by performances in Dublin, London, New York and Luxembourg.

[6] The Hunger, about the Great Irish Famine, premiered in June 2016 at a concert performance in Washington DC, and in a staged production in St. Louis and at the Brooklyn Academy of Music, all with the orchestra Alarm Will Sound.