Dennis O'Brien (police officer)

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 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 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.