Lacy, William de (p.1180–1233), son of Hugh de Lacy I (qv) and his second wife, a daughter of Ruaidrí Ua Conchobair (qv), king of Connacht, was originally granted lands in Meath as part of the lordship of his brother, Walter de Lacy (qv). He also held the castles of Kilmore and Belturbet on the border with Breifne but was deprived of these lands by King John (qv) in 1210 for his support of his other brother, Hugh de Lacy II (qv).
William probably surrendered to the king at Carrickfergus in 1210 and was placed in the custody of the sheriff of Gloucester for the next five years. He was released in 1215 and returned to Ireland, where he took over the governance of the lordship of Meath while his brother remained in England. He also appears to have attempted the restoration of de Lacy authority in the earldom of Ulster. This led to conflict with other members of the Anglo-Irish community, and in 1217 de Lacy was ordered to turn over the castles of Carlingford and Rath to the justiciar, Geoffrey de Marisco (qv), while Walter de Lacy promised compensation to the king for his brother's actions.
When Walter returned to Ireland in 1220, the brothers began to expand the de Lacy lordship into Breifne, where William was to have a lordship carved out of the O'Reilly kingdom; he captured Ó Ragallaig's crannog in Lough Oughter and used it as his base for the next four years. About this time (c.1223) William married a daughter of Llywelyn ab Iorwerth, prince of North Wales. He actively supported the rebellion led by Hugh de Lacy in 1224, and this proved to be his undoing. His Irish enemies, Cathal Ó Ragallaig (O'Reilly) (qv) and Aed Ó Conchobair (qv), king of Connacht, made peace with the English, and with the aid of a force sent by the new justiciar, William Marshal II (qv), earl of Pembroke, were allowed to push de Lacy out of Breifne. By 1226 de Lacy's castles at Lough Oughter and Kilmore were destroyed by Ó Ragallaig.
De Lacy was eventually rehabilitated, as in 1227 he received an annuity of £20 from the king. He also appears to have remained active in the border skirmishing between Meath and Breifne, as his death is recorded in the annals in 1233, where he is said to have died of wounds taken in a raid into Breifne.