He also participated in masterclass and lessons with composers including Richard Barrett, Chaya Czernowin, Brian Ferneyhough, Jonathan Harvey, Alvin Lucier, and Tristan Murail, among others.
[2] Cassidy's music exhibits a radical approach to parametric organisation in composition, especially in his solo works, in a manner that he describes as to do with "decoupling".
He is concerned to explore the possibilities of fracturing between different musical parameters, and to defamiliarise aspects of traditional performance practice.
Cassidy's work has been performed by a wide range of leading contemporary music specialists, including ELISION Ensemble, Ensemble SurPlus, the Ictus Ensemble, the JACK Quartet, the Kairos Quartet, Quatuor Diotima, ensemble recherche, Mieko Kanno, Garth Knox, Ian Pace, Christopher Redgate, Carl Rosman, and Peter Veale, at many venues and festivals including the Gaudeamus International Music Week (where he was a Jurors Prize nominee, in 2002 and 2004), the Huddersfield Contemporary Music Festival, the Bludenzer Tage zeitgemäßer Musik, the Bienal Internacional de Musica y Tecnología (Mexico City), the Samtida Musik Stockholm, the festival June In Buffalo, and the ISCM World Music Days (Zagreb 2005), as well as being broadcast by radio stations in Britain, France, Germany and Austria.
He has also worked recently as a CD producer, particularly for EXAUDI Vocal Ensemble in their discs of the music of Michael Finnissy, Christopher Fox, and Howard Skempton, which were released on the NMC label.