Denis Gwynn was born on 6 March 1893, the third son of Stephen Gwynn, the Irish patriot, writer and Irish Parliamentary Party Member of Parliament. His mother was Mary ('May') Louisa Osborn Gwynn; his parents were first cousins.. The middle name Rolleston was derived from Denis Gwynn's great grandmother Catherine Rolleston, who married his great grandfather John Gwynne.
Along with his mother and siblings, but not his father, Denis Gwynn was received into the Roman Catholic Church in 1902. He was educated at St. Enda's School Rathfarnham, Clongowes Wood College and at University College Dublin where he graduated BA (1914), MA (1915) and D.Litt. (1932).
During World War I, in 1916, Gwynn enlisted in the Royal Munster Fusiliers. He served on the Western Front in France from 1916 to 1917, but was then invalided home and worked for the remainder of the war at the British Ministry of Information.
After the war Gwynn worked as a journalist. He became assistant editor of the periodical Everyman in London, joined the National Press Agency, worked for a while as a reporter in Brittany and Paris, then in 1922 returned to London where he was active for many years as a journalist specialising in Irish Catholic issues. He was on the editorial staff of the Westminster Gazette and edited the Dublin Review from 1933 to 1939.
In 1948 Gwynn returned to Ireland and took up the post of research professor of Modern Irish History at University College Cork. He remained in this post until his retirement in 1962. Professor Gwynn also acted as editor of the Cork University Press (1954-1962) and wrote a regular column called Now and Then in the Cork Examiner. He wrote several works of history and biography and was a contributor to Encyclopedia Britannica.
Denis Gwynn was married to Alice Trudeau (born in the US, 1904), the only daughter of Dr Edward Livingston Trudeau and Hazel Martyn, an American beauty who later became the wife of Irish artist John Lavery; for many years Hazel Lavery's face, as drawn by Lavery, featured on Irish banknotes.