Studied at University College Dublin and for a Master’s degree under Ludwig Mies van der Rohe at the Illinois Institute of Technology, graduating 1959. Worked with Mies van der Rohe until returning to Ireland to work at the school of architecture in UCD, before establishing Cathal O’Neill Associates, Architects in 1961. Apart from his building projects, he has been primarily involved in architectural education since 1961: he held the Chair of Architecture at University College Dublin for twenty-three years (1973-96) and lectured and served as visiting critic and external examiner throughout the world.