[1][3][4] After his graduation he taught at Belfast Royal Academy and completed an MA degree under the supervision of T. W. Moody.
His MA dissertation was published by Faber under the title Protestant Dissent in Ireland 1687–1780 and was the second volume in the 'Studies in Irish History' series, of which Moody was co-editor.
In 1958 Queen's University awarded him a personal chair in Irish history, which he held until he retired in 1975 and assumed emeritus status.
[4] The Making of Modern Ireland, published in 1966, is considered his master-work and it became a standard textbook in schools and colleges.
[4] The book was groundbreaking in that it presented the findings of a critical generation of Irish historians and was, according to David Quinn, "not only learned but cool, objective, unimpassioned and yet always alive and compassionate as well".