Attached to C Company, 4 Battalion, Dublin Brigade, Irish Volunteers, Denis O'Brien took part in the Rising with his two brothers, Laurence (MSP34REF1336) and Patrick (DP2196). He served in Roe’s Distillery, James’s Street, for the first two days and then in Jameson Distillery, Marrowbone Lane, until the surrender on 30 April. Denis was imprisoned in Dublin until 6 May 1916 but released on account of his young age (16).
During the War of Independence, he served as a First Lieutenant and took part in the execution of two members of the Auxiliary Division on Crane Street. Taking the anti-Treaty side in the Civil War, he was captured at the surrender of the Four Courts on 30 June 1922. His brother Patrick was killed during fighting in County Wexford a few days later. Denis was interned in Mountjoy Jail and Newbridge Camp, County Kildare, until February 1924.
Denis O’Brien joined An Garda Síochána on 9 August 1933 following the election victory of Éamon de Valera’s anti-Treaty Fianna Fáil party. He rose to the rank of Detective Sergeant in Dublin Castle’s Special Branch.
Former anti-Treaty IRA men, including O’Brien, were initially recruited into Special Branch in the early 1930s to combat Eoin O’Duffy’s fascist Blueshirts, which were composed largely of former National Army soldiers. From the late 1930s onwards, Special Branch increasingly focused on the remnants of the IRA, which included many veterans of the War of Independence and the anti-Treaty campaign, along with younger recruits.
Historian Donnacha Ó Beacháin states in Destiny of the Soldiers that O’Brien began “hunting down” his former IRA comrades with the “zeal of a convert” within a few years of joining Special Branch. It was alleged by Conor Foley in Legion of the Rearguard: Republicanism, Nationalism and the Irish Free State (1992) that O’Brien “was well known for brutality in the interrogation room and his trigger-happy attitude during raids” in the early 1940s.
On 9 September 1942, Denis O’Brien was ambushed by an IRA unit at the gateway to his home at Thomond House, Ballyboden Road, Rathfarnham, South County Dublin. He was shot and killed by three men armed with revolvers and Thompson submachine guns. He was 43.
Source: Jimmy Wren, The GPO Garrison Easter Week 1916: A Biographical Dictionary (2015).
