McCullough, Denis (1883–1968), revolutionary and businessman, was born 24 January 1883 in Divis St., Belfast, son of Daniel McCullough, publican, and Margaret McCullough (née Magee), of Quilly, Dromore, Co. Down. Educated locally by the Christian Brothers, he served an apprenticeship as a piano tuner with Messrs Crane & Sons, High St., and was employed with Charles Hardy & Son, Anne St., before establishing his own businesses at 8 and 12 Howard St.
A member of the Gaelic League in the 1890s and a founder of the first hurling club in Belfast, he was sworn into the IRB by his father c.1900 and purged the Belfast circles of their older members (including his father), replacing them with younger men, including Seán MacDermott (qv). Chairman of the Ulster provincial council from 1905, and Ulster representative on the supreme council 1907–16, he was responsible, along with Bulmer Hobson (qv), MacDermott, and Tom Clarke (qv) for the greater militancy of the IRB after 1907. In 1905, with Hobson, he established the Dungannon Clubs, a non-sectarian, republican, separatist organisation named after the eighteenth-century Irish Volunteers' Dungannon convention, which eventually merged with Cumann na nGaedheal and the National Council of Arthur Griffith (qv) to form Sinn Féin. A member of the Irish Volunteer executive in 1913, he established several Volunteer corps in Belfast. Disobeying an order to leave Ireland in 1915 under the Defence of the Realm Act, he was imprisoned (August–November 1915), and was elected president of the IRB supreme council in September 1915. However, he was largely kept in the dark about plans for the Easter rising, during which his efforts to join the uprising led by Liam Mellows (qv) in Connacht failed, and he was imprisoned in Richmond barracks, Knutsford, Frongoch, and Reading before his release on 6 August 1916. Having left the IRB after the rising, feeling he had been sidelined, he remained an ordinary Volunteer during the war of independence, much of which he spent in prison: Belfast, Arbour Hill, and Gloucester (May 1918–March 1919); Belfast (April–July 1920); Belfast and Mountjoy (October 1920–January 1921); and Belfast, Holywood, and Ballykinlar (January–December 1921). A supporter of the Anglo–Irish treaty, he undertook between March and June 1922 a mission to the USA, aiming to reunite the warring Irish-American factions, in particular Clan na Gael (at which he was unsuccessful); convince them to abstain from Irish party politics; establish control of dáil funds for the provisional government; and get publicity on the Ulster question. A member of Belfast corporation (1918–22) and elected MP for Tyrone South in the 1918 general election, he was defeated as a Sinn Féin candidate in Belfast West in the election for the first Northern Ireland parliament in 1921. Travelling to Germany in 1922, he convinced Col. Fritz Brase (qv) to become director of the Army School of Music. He was elected Cumann na nGaedheal TD for Donegal in a 1924 by-election, but resigned from the party in 1925 in protest at the government's handling of the boundary commission. He retired from politics in 1927.
Forced by the Belfast boycott to move his business to Dublin, he had to rebuild his premises at 56 Dawson St., after their destruction by an anti-treaty IRA landmine in December 1922, and he continued in business as McCulloughs Ltd, merging in 1967 with Pigotts Ltd to form McCullough Pigott. Later he served as vice-president of the Royal Irish Academy of Music, and supported the establishment of the Gate Theatre in 1928. An influential figure in Irish industrial development after independence, he was vice-president of the Federation of Irish Manufacturers, Irish representative at an inter-parliamentary commercial conference in Rio de Janeiro (1927), and employers' adviser to the Irish delegation at the twentieth session of the International Labour Conference in Geneva (1936). Chairman of the New Ireland Assurance Company (1922–64), and of Clondalkin Paper Mills, James Crean & Sons (1936) Ltd., Moore Clothing Ltd, M. Duan & Co. Ltd, and Weartex Ltd, he was a director of the Irish National Insurance Company Ltd, Bowater's Irish Wallboard Mills, C. B. Paper Sacks Ltd, and Drimnagh Paper Mills Ltd.
He married (16 August 1916) Agnes, daughter of John Ryan, farmer, of Tomcoole, Co. Wexford, sister of Dr James Ryan (qv), and sister-in-law of Richard Mulcahy (qv) and Seán T. O'Kelly (qv), whom he had met while she was teaching in the Dominican Convent, Belfast. They had four sons and two daughters and lived at 4 Leeson Park (1921–35) and 12 Oakley Road, Ranelagh (1936–68). He died 11 September 1968 in Dublin, leaving an estate valued at £46,379. His papers are in the UCD archives department, and there is a portrait of him by Seán O'Sullivan (qv), RHA, in the New Ireland Assurance Company.