On 20 March 1920, Oswald Swanzy was in charge of a group of masked RIC policemen who entered the home of Tomás Mac Curtain, Lord Mayor of Cork, and killed him. Mac Curtain was also the leader of Cork No. 1 Brigade of the IRA.

On 17 April 1920, a coroner’s inquest was held into the death of Mac Curtain. The jury returned a verdict of murder against RIC DI Oswald Swanzy, British Prime Minister, David Lloyd George, The Lord Lieutenant of Ireland John French, Viscount French, and the Inspector General of The Royal Irish Constabulary T.J. Smith.

The British, in an effort to protect Swanzy from IRA reprisals, transferred Swanzy from Cork to Lisburn.

Collins’s vast intelligence network, with the help of RIC Sergeant Matt McCarthy who had provided Collins with information in the past, was able to track down DI Swanzy in Lisburn. Collins dispatched a special hit team comprised of members of the First Battalion, Cork No. 1 Brigade to kill him. They achieved their objective on 22 August, as Swanzy was leaving Christ Church Cathedral, Market Square, Lisburn.

It is widely believed that Mac Curtain’s personal handgun was used to kill Swanzy. He was buried in Mount Jerome Cemetery on 25 August 1920.

Catholic residential areas of Lisburn were burned in revenge by local loyalists. Several people were later prosecuted for the burnings. Loyalists attack Catholic areas of Belfast in reprisal. A total of 33 people died over the next ten days in sectarian rioting and shooting in the city.