Sarsfield, Sir William (1520–1616), politician, was the son of alderman John Sarsfield of Sarsfieldstown, Co. Meath. A wealthy landowner, he was deeply involved in the civic life of Dublin, beginning his connection with the municipal government in 1554–5, when he served as sheriff. In 1560 he was co-opted an alderman of the city, and in 1566–7 he served as mayor, as had his father and elder brother Patrick before him; he was knighted in 1566, during his year of office, for raising a band of men to defend the town of Drogheda, then threatened by Shane O'Neill (qv). He subsequently served as treasurer in 1569–70. Sir William was also active in the affairs of the wider Pale area, serving as sheriff of Co. Dublin in 1572 and again in 1582. In the period 1572–4, he was a member of a number of commissions aimed at defending the county, and in March 1577 he was appointed to a commission to survey and shire Co. Wicklow. He was again active in the defence of the Pale during the rebellions led by James Eustace (qv), 3rd Viscount Baltinglass (1581–2), and William Nugent (qv) (1582), even taking to the field against Fiach MacHugh O'Byrne (qv), Baltinglass's ally, during the winter of 1581–2.
Sarsfield's family estate, which he had inherited by 1566, was at Lucan, Co. Dublin, while his main residence was on High Street, Dublin. As a protestant he enjoyed the trust of the government and benefited accordingly: he was granted a twenty-one-year lease of the preceptory of Tully, Co. Kildare, on 14 July 1569, and on 28 February 1592 he was appointed seneschal of the queen's manors of Newcastle Lyons, Esker, Saggart, and Crumlin. He married Margaret (d. 16 May 1604), daughter of Andrew Tyrell of Athboy, Co. Meath, with whom he had three sons, and three daughters. Their sons were: John (d. 24 January 1615), of Turnings, Co. Kildare, who married Margaret, daughter of Sir Lucas Dillon (qv), chief baron of the exchequer; Patrick (d. 9 January 1630), of Tully, Co. Kildare, who married Cicilie, daughter of Matthew Fitzhenry, of Kilcavan, Co. Wexford; and Simon (d. ante 1627), of Morristown-Moynagh, Co. Kildare. Of their daughters, Katherine (d. 21 December 1615) married firstly Sir Robert Dillon (qv) (d. 1597), of Riverstown, Co. Meath, chief justice of the common bench, and secondly Sir Christopher Bellew, of Bellewstown, Co. Meath, and Mary married Christopher Bath, of Rathfegh, Co. Meath. Sir William died 17 December 1616 and was buried at Lucan.