[2] McDonagh was born in Tralee, but grew up in Sandycove, County Dubin.
He attended the Presentation Brothers school in Glasthule and joined the Irish civil service as a clerical officer.
[2] An Anglo-Irish Division of the Department of Foreign Affairs was set up for the first time in early 1972 and McDonagh became its head.
[2] In 1983 he was appointed as Ireland's Permanent representative to the United Nations in New York and served in that post until his retirement in 1989.
McDonagh amassed a library of over 1,000 diplomatic biographies in several languages which his family donated to the Irish Department of Foreign Affairs after his death.