Gywnn, Denis Rolleston (1893–1971), journalist and historian, was born 6 March 1893, third son among four sons and two daughters of Stephen Lucius Gwynn (qv), writer and MP, and his wife and first cousin Mary Louise, daughter of the Rev. James Gwynn of Dublin and Bath. On his mother's conversion (1902) to Roman catholicism, Denis, his brother Aubrey (qv), and their siblings were brought up as catholics. He was educated at St Enda's, the celebrated school of P. H. Pearse (qv), where he was taught by Pearse himself and by Thomas MacDonagh (qv); at Clongowes Wood College (1903–8); and at UCD (BA 1911, MA 1915). He edited the weekly New Ireland in association with P. J. Little (qv), later a Fianna Fáil government minister. He worked at the British ministry of information during the Great War, and later claimed that a senior ministry official, G. H. Mair, had photographed the Casement ‘Black diaries’ for distribution to foreign embassies and journalists in 1916. In 1917–18 he saw active service as an officer in the Munster Fusiliers and was invalided home.
In postwar years he became an experienced journalist, working in France for three years and holding various editorial positions in the weekly Everyman, Review of Reviews, and Westminster Gazette. He was London editor of the Freeman's Journal in its last phase before it ceased publication in 1924. He had a professional association with the Catholic Times and The Universe, became the literary director of Burns, Oates & Washbourne, and edited the quarterly Dublin Review from 1933 to 1939. He wrote for leading journals and was the chief contributor to the Encyclopaedia Britannica on Irish historical subjects.
During the second world war he retired for health reasons to Hampshire, where he worked a small farm. Meanwhile, his published historical work earned him a D.Litt. from the NUI in 1945. He was also a member of the RIA, a member of the Irish Academy of Letters, and a fellow of the Royal Historical Society.
In 1946 he was appointed research professor of modern Irish history at UCC. He played a prominent part in the cultural life of UCC and Cork city, and encouraged the formation of a graduates' association. He worked energetically to organise art exhibitions, to promote local artists, and, through his wide network of contacts, to attract notable literary, artistic, and political figures to Cork as speakers. For many years he wrote an occasional column of social commentary, ‘Now and then’, for the Cork Examiner. Obituaries were a special feature of this column and when he wrote about Sean Moylan (qv), minister for education 1951–4, he asserted that Moylan had supported the idea of a dictionary of Irish biography, which Gwynn and M. J. MacManus (qv) had been interested in promoting.
He became general editor of Cork University Press in 1955 and in that capacity made substantial contributions to the College periodical, UCC Record, including valuable information on the early history of QCC. He retired from UCC in March 1963, the year he married Alice McEnery, widow of John A. McEnery of Co. Kilkenny, and daughter of Edward Trudeau of New York, surgeon, and his wife Hazel (afterwards Lady Lavery (qv)). The Gwynns lived in Dublin until his death on 10 April 1971.
Gwynn was a prolific, and sometimes prolix, writer with an impressively lengthy number of publications. They include The catholic reaction in France (1924), The Irish Free State 1922–27 (1928), A hundred years of catholic emancipation (1929), Edward Martyn and the Irish revival (1930), The second spring 1818–52 (1942), The Vatican and war in Europe (1936, 1940), Lord Shrewsbury, Pugin and the catholic revival (1946), and The history of partition, 1912–1925 (1950). He wrote biographies of John Keogh (qv) (1920), Roger Casement (qv) (1920), Cardinal Wiseman (1929), Éamon de Valera (qv) (1933), Daniel O'Connell (qv) (1947), and, most importantly, John Redmond (qv) (1932) – a comprehensive but largely uncritical treatment. He was especially interested in the Young Ireland period, and in his Young Ireland and 1848 (1949) his pietas towards his great-grandfather, William Smith O'Brien (qv), was particularly evident. He made extensive use of source material such as the Redmond and Smith O'Brien papers but did not always document his references in accordance with the requirements of modern scholarship.
Despite, or because of, his early exposure to the educational philosophy of Pearse and St Enda's (Pearse singled him out for praise as a star pupil), Denis Gwynn's cultural and political sympathies were those of Old Ireland, if not indeed Anglo-Ireland, rather than Irish-Ireland. Similarly, he paid special attention to the history of English catholicism, as in his important essay on ‘England and Wales’ in A history of Irish catholicism, ed. P. J. Corish, vi, fasc. 1 (1968).