Detective Sergeant Denis O'Brien (17 June 1899 – 9 September 1942), sometimes called "Dinny O’Brien", was a veteran of the Irish War of Independence and the Irish Civil War. He joined the Garda Síochána in 1933 and was killed by the Anti-Treaty IRA in 1942. Early life Denis O'Brien was born at 3 Boatman's Lane, near Cork Street, Dublin, the son of Patrick O'Brien, a labourer, and Mary Anne O'Brien, née Kane. The O'Brien family were still living at Boatman's Lane at the time of the 1901 census, but by the 1911 census they had moved to Pim Street, near Usher's Quay, Dublin. He was educated at James Street school by the Congregation of Christian Brothers. As a 17-year-old, he fought in the 1916 Easter Rising with the Marrowbone Lane Garrison of the Irish Volunteers. Briefly imprisoned by the British Army at Richmond Barracks, O'Brien was released on account of his age. He joined the Irish Republican Army in 1917, eventually succeeding his brother Patrick as O.C. ("officer commanding") 'C' Company, 4th Battalion, Dublin Brigade. He commanded his company through the whole period up until the Anglo-Irish Treaty. During the Irish Civil War, Denis and his brothers joined the Anti-Treaty IRA and fought in the Four Courts. After his capture during the Battle of Dublin, Denis was interned at the Curragh Camp until 1924. He later served as an accountancy clerk with the Electricity Supply Board. He married Annie Cooney at St James' Catholic Church, Dublin on 12 April 1926. Police career In 1933, Éamon de Valera, the new President of the Executive Council of the Irish Free State, issued a call for IRA veterans to join the Gardaí. O'Brien joined the Garda Síochána on 9 August 1933 (with registration number 8288), and subsequently entered the Detective Branch section headed by Eamon Broy. Broy, a former Detective Sergeant in the G Division of the Dublin Metropolitan Police, had spied for Michael Collins during the Irish War of Independence. Among Irish republican legitimists, however, O'Brien and his colleagues were referred to as, "The Broy Harriers." O'Brien was promoted to Detective Sergeant on 15 October 1937 and remained in the Gardaí when de Valera introduced a more Republican constitution in 1937 and abolished the Oath of Allegiance to the British Monarchy. World War II During the Second World War O'Brien was a Detective Sergeant in the Special Branch Division, which had its