Walter (d. 1241), the elder son, by Rose of Monmouth, inherited the Welsh estates, and, having tasted of king John's vengeance during the Irish expedition of 1210 (he had become involved in the affairs of Llywelyn and William de Breos), thereafter appears as a staunch supporter of the Crown, being among the marcher lords on the side of John in the crisis of 1215, and on that of Henry III during the Marshal rising of 1233. Hugh, 1st earl of Ulster, proved less amenable; he spent many years in exile and may, for a short time, have been a fugitive in Wales.

William (d. 1233), son of Hugh II by a daughter of Roderick O'Connor, m. Gwenllian, daughter of Llywelyn ap Iorwerth, who, like her brother Gruffydd ap Llywelyn (q.v.), was the child of an irregular union contracted before Llywelyn's marriage with Joan (q.v.). During a long widowhood (she d. in 1281), she may have resided at Llys Gwenllian on the royal manor of Ystrad Owain in the commote of Cinmeirch, supported from lands in that neighbourhood granted to her by her father, and from her dower lands in Ireland.

Owing to the eventual extinction of the male line, the De Lacy property in the March passed through marriage in the course of the 14th cent. to the Mortimers, earls of March. Members of the family were great benefactors of Llanthony and S. Peter's, Gloucester.