Dennis O ' Brien ( policeman ) Detective Sergeant Denis O ' Brien , Registration Number : 8288 ( 17 June 1899 – 9 September 1942 ) , often 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 . Early life . Denis O ' Brien , born at 8 Pim Street , Dublin 8 , was educated at James Street school by the Congregation of Christian Brothers . 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 officer commanding ' C ' Company 4th Battalion , Dublin Brigade . He commanded his Company through the whole period up until the Anglo - Irish Treaty . After the Treaty , he and his brothers joined the Anti - Treaty IRA and fought in the Four Courts . During the Battle of Dublin , Denis was captured and interned at the Curragh Camp until 1924 . He later served as an accountancy clerk with the Electricity Supply Board . 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 . 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 and known as the " Broy Harriers " . Promoted to Detective Sergeant on 15 October 1937 , O ' Brien remained in the Gardaí when de Valera introduced a more anti - monarchist 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 gun s , 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 Staff Charlie 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 , and two daughters , Fiona and Eithne . 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 . After Kerins ' fingerprints were identified on the bicycle which was left at the crime scene , he was found guilty , and sentenced to death by hanging . The sentence was carried out by British chief executioner Albert Pierrepoint at Mountjoy Prison on 1 December 1944 , in spite of numerous calls for clemency . Archie Doyle , who is also alleged to have in involved in the killing of O ' Brien , died in 1980 .