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. Many pro-Treaty veterans of the Civil War had lost their jobs in the Irish police and military after de Valera won took power. De Valera hoped to fill their places with men who shared his views on the Anglo-Irish Treaty. Many Anti-Treaty veterans who answered his call regarded this as an opportunity to continue fighting their Civil War foes. O'Brien joined An Garda Síochána on 9 August 1933, and subsequently entered the Detective Branch section headed by Eamon Broy. Broy, a former Detective in the G Division of the Dublin Metropolitan Police, had spied for Michael Collins during the Irish War of Independence. To hardline Irish Republicans, 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 headquarters at Dublin Castle. The Special Branch Division was then largely tasked with hunting down foreign spies and members of the IRA, who were interned in the Curragh Camp. De Valera's government regarded the collaboration of some elements of the IRA with the intelligence services of Nazi Germany as a threat to Irish neutrality. According to historian Tim Pat Coogan, "An iron gloved approached to the IRA was the order of the day with vigorous raids and interrogations. As a result, relations between individual IRA men and the Special Branch became understandably strained, and the IRA, in its shattered and disorganized condition, came to regard the Special Branch as a greater enemy than the British Crown." At the time, the IRA regarded the Irish Free State as a de facto extension of the British Empire. Therefore, Irishmen who served the Free State were regarded as traitors.
Assassination
At 9:45 am on 9 September 1942 at Ballyboden, Rathfarnham, County Dublin, O'Brien left his house and began getting into his car. As was customary among Gardaí, Detective Sergeant O'Brien was unarmed. Three IRA men, wearing trenchcoats and armed with Thompson sub machine guns, came up the drive and opened fire. The shots from the Thompson smashed the windows of his car, wounding him. He alighted and ran for cover to the gate but before reaching it, he was shot by a single round to the head. Two of the assassins wrapped the Thompsons in their trenchcoats, mounted their bicycles, and rode towards Dublin. Future IRA Chief of StaffCharlie Kerins left on foot, leaving his bicycle behind. According to author Tim Pat Coogan, "The shooting greatly increased public feeling against the IRA, particularly as the murder was carried out almost in full view of his wife. As she held her dying husband, she watched his assailants cycling past." Detective Sergeant Denis O'Brien was survived by his wife, Anne Cooney, and two daughters.
Aftermath
Two years later, Kerins was arrested in a pre-dawn raid and tried by court-martial for the murder of Detective Sergeant O'Brien. At a special military tribunal in Collins Barracks, Dublin, Kerins was formally charged on 2 October 1944 for the "shooting at Rathfarnham of Detective Dinny O'Brien". According to Coogan,
"At the end of his trial, the president of the Military Court delayed sentence until later in the day to allow Kerins, if he wished, to make an application whereby he might have avoided the capital sentence. When the court resumed, Kerins said: "You could have adjourned it for six years as far as I am concerned, as my attitude towards this Court will always be the same." He thus deprived himself of the right to give evidence, to face cross examination, or to call witnesses.
