Just to add to chanteyranger's info on Seán Ó Riada -- I got the information below from the Contemporary Music Centre Ireland website, which you can find here:

Seán Ó Riada was born in 1971 in Cork. He studied music at University College, Cork with Aloys Fleischmann and received his B.Mus in 1952. He was Assistant Music Director at Radio Éireann between 1954 and 1955, and was Music Director at the Abbey Theatre from 1955 to 1962. He lectured in music at University College Cork from 1963 until his death in 1971.

Although best known for his work with Irish traditional music and the group, Ceoltóirí Chualann, Ó Riada wrote a number of original compositions which incorporated modern compositional techniques. These include a series of orchestral works given the Greek title Nomos, written between 1957 and 1966; the pastoral elegy, The Banks of Sullane (1956) and Four Hölderlin Songs (1956). He also wrote music for several films, including Mise Éire and Saoirse.