William Sarsfield was an Irish landowner of the seventeenth century. He was the elder brother of the Jacobite soldier Patrick Sarsfield. He was of both Gaelic (he was a grandson of Rory O'Moore) and Old English descent. The Sarsfields had come to Ireland during the Norman conquest of the twelfth century and had become leading figures of The Pale. His great, great-grandfather Sir William Sarsfield served as Mayor of Dublin and purchased an estate at Lucan Manor. Like most of the Old English community, the Sarsfield remained Roman Catholic. His father Patrick Sarsfield senior took part in the Irish Rebellion of 1641, but like other Irish Confederates continued to swear allegiance to Charles I. He assisted royal forces against the English republicans during the Cromwellian conquest of Ireland