[6] 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 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.
The Special Branch Division was then largely tasked with hunting down foreign spies and members of the IRA, who were interned and held without trial in the Curragh Camp.
De Valera's government regarded the collaboration of the IRA with the intelligence services of Nazi Germany as a serious threat to Irish neutrality and its national security.
Three IRA men, wearing trenchcoats and armed with Thompson submachine guns, lay in wait for him as he drove out his drive and opened fire.
The fatal shot was from a volley from a second Thompson gun fired by another IRA man secreted across the Ballyboden Road adjacent to Kerr's Sawmill.
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.
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".
[12] The sentence was carried out by British chief executioner Albert Pierrepoint, who was regularly employed for such occasions by the Irish State, at Mountjoy Prison on 1 December 1944, in spite of numerous calls for clemency.