Hayes St Leger, 4th Viscount Doneraile (1 October 1818 – 26 August 1887) was a member of the Peerage of Ireland who was an Irish representative peer in the British House of Lords between 1855 and his death.^[1]

Doneraile was the son of Hayes St Leger, 3rd Viscount Doneraile and Lady Charlotte Esther Bernard. He married Mary Ann Grace Louisa Lenox-Conyngham, the daughter of George Lenox-Conyngham and Elizabeth Holmes, on 20 August 1851.^[2] Their daughter Emily Ursula Clare St Leger married Bernard FitzPatrick, later 2nd Baron Castletown; their other children, Hayes and May, died as infants in 1852 and 1867.^[2]

Doneraile succeeded his father as Viscount in 1854.^[2] After Viscount Lorton's death later that year, the Irish peers elected Doneraile a representative in his place on 2 May 1855,^[1] and he took the oaths at Westminster on 10 May.^[3] He was a Conservative and a member of the Carlton Club.^[4] Rarely present in Parliament, he voted by proxy for the Earl of Derby's 1857 resolution condemning the conduct of the Second Opium War.^[5] He was High Sheriff of County Cork for 1845,^[6] a Deputy Lieutenant for County Cork,^[7] and an Honorary Colonel of the North Cork Militia^[8] and the 9th Battalion, King's Royal Rifle Corps.^[2] Doneraile was a prominent and frequent fox hunter, and was Master of Fox Hounds at the Burton Hunt in Lincolnshire and President of the Duhallow Hunt in Mallow.^[9]

Doneraile kept a pet vixen, which bit him and his coachman, Robert Barrer, on 13 January 1887 and was found to have rabies.^[9] At the urging of his son-in-law, Castletown, Doneraile and Barrer travelled to the Pasteur Institute in Paris to receive an experimental post-exposure vaccine.^[9] Louis Pasteur was travelling and there was a delay before he was reached in Naples. Doneraile's doubts over the vaccine's risk caused him to vacillate further before allowing Jacques-Joseph Grancher to administer two doses, on 24 January and 21 February.^[10][11][12] While Barrer survived, Doneraile began to feel unwell on 22 August and to suffer convulsions and delirium on 25 August, dying the following morning at home in Doneraile Court.^[9][13] The relatively late and mild onset of symptoms was attributed at the time to the partial effect of the vaccine.^[13] An apocryphal tale is that Doneraile was deliberately smothered as a mercy killing.^[12][14] The death of such a notable fed the controversy over the vaccine.^[15] Pasteur in the British Medical Journal blamed its failure on the delay in starting and the fact that Doneraile accepted only a "simple treatment" rather than the recommended "intensive course".^[10] Victor Horsley said Doneraile "refused to go through the treatment ordered";^[15] later accounts suggest Doneraile cut short his treatment through boredom or impatience.^[16][12]