FITZWILLIAM, SIR WILLIAM lord deputy of Ireland, was the eldest son of Sir William Fitzwilliam (d. 1576) of Milton, Northamptonshire, where he was born. In he became vice-treasurer of Ireland and a member of the Irish House of Commons; and between this date and 1571 he was (during the absences of Thomas Radclyffe, earl of Sussex, and of his successor, Sir Henry Sidney) five times lord justice of Ireland. In 1571 Fitzwilliam himself was appointed lord deputy, but like Elizabeth's other servants he received little or no money, and his period of government was marked by continuous penury and its attendant evils, inefficiency, mutiny and general lawlessness. Moreover, the deputy quarrelled with the lord president of Con naught, Sir Edward Fitton (1527-1579), but he compelled the earl of Desmond to submit in 1574. - Returning to England in he was governor of Fotheringhay Castle at the time of Mary Stuart's execution. In 1588 Fitzwilliam was again in Ireland as lord deputy, and although old and ill he displayed great activity in leading expeditions, and found time to quarrel with Sir Richard Bingham (1528-1599), the new president of Connaught. In he finally left Ireland, and five years later he died at Milton. From Fitzwilliam, whose wife was Anne, daughter of Sir William Sidney, were descended the barons and earls Fitzwilliam.
See R. Bagwell, Ireland under the Tudors, vol. ii. (1885) .