Dennehy’s music is marked by a sonic and rhythmic intensity and a kind of volatile, dirty tonality that travels in and out of an overtone-based focus.
His fascination with the way time and light stretch and contract through the seasons of the year in his native Ireland, and the psychological impact of such phenomena, has often influenced the structure of his music generally. The intersection between words and music, and the vocal sean-nós (old style) tradition has exerted a strong pull over him too since he was a child attending informal get-togethers at his grandmother’s house in Kerry. "There'd be long, all-night sessions in my grandmother's house with singing and poetry, and people remembering 30-stanza poems," Dennehy told NPR. Returning to Ireland after studies abroad, principally at the University of Illinois in the US, Dennehy founded Crash Ensemble, Ireland’s now-renowned new music group, in 1997. Alongside the singers Dawn Upshaw and Iarla Ó Lionáird, Crash Ensemble features on the 2011 Nonesuch release of Dennehy’s music, entitled Grá agus Bás. Other releases include a number by NMC Records in London, Bedroom Community in Reykjavik and Cantaloupe in New York. He joined the music faculty at Princeton University in 2014, and now lives in America. In recent years, Dennehy has completed two operas with Enda Walsh, The Last Hotel and The Second Violinist. He has also written a kind of “docu-cantata”, The Hunger, for Alarm Will Sound.