He played a prominent role in suppressing the Irish Rebellion of 1798, which included personally killing Society of United Irishmen leader Lord Edward FitzGerald, who Sirr alleged had been resisting arrest.

In 1796, upon the formation of yeomanry in Dublin, he volunteered his services, and was appointed acting town-major or head of the police, and was thenceforward known as the chief agent of the Castle authorities.

In the months prior to their rising in May and June 1798, he was prominent in the arrests of Peter Finnerty, the editor of their Dublin paper, the Press, on 31 October 1797, and of their leaders Thomas Russell and the popular Lord Edward Fitzgerald.

[6] The wound that Sirr inflicted upon Fitzgerald is commonly supposed to have been fatal, although an inquest found on the evidence of the attending surgeon that his death on 4 June resulted from "water on the chest".

[7] As a result of the arrest, pressure mounted within the United Irish organisation to rise before its leadership structure in Dublin was entirely disrupted and its arms stores elsewhere were confiscated.

Sirr and his colleague had then subjected Hevey to wrongful arrest, imprisonment incommunicado, extortion of goods and money, and condemnation to death by hanging.

He continued to discharge his duties as town-major until 1826, when he retired upon full pay, and in consideration of his public services was allowed to retain his official residence in Dublin Castle.

[11] Niles' Register of 24 March 1821 remarks that "Several persons have been arrested at a public house in Dublin, by major Sirr, charged with being engaged in a treasonable meeting, and committed to prison... We thought that this old sinner, given to eternal infamy by the eloquence of Curran, had gone home".

Five-and-thirty years had intervened between the pillage of the Catholic leader’s house, and the lodging of its owner in Newgate – and the giving of his vote to send another to the imperial parliament.

"[18] In Master and Commander by Patrick O'Brian, the character Stephen Maturin refers to the Rebellion as having spawned "a vile race of informers and things like Major Sirr".