Ellis suggests that Walter de Lacy was the brother of Ilbert de Lacy, highlighting that Walter held a "knight’s fee in Lassy and Campeaux" in Normandy from Odo Bishop of Bayeux and that "this fief was afterwards held in parage by the heirs of Walter and Ilbert". This is supported by the two documents, dated to  and Sep 1146 both quoted below, which list fiefs of the bishop of Bayeux in the duchy of Normandy. Ellis shows the supposed brothers in his genealogical table as born from different mothers. Presumably he bases this on Emma naming only her son Ilbert in the charter which is quoted above. However, there could be many explanations for the omission of Walter from the document and, assuming that the two were brothers, it seems probable that Emma was also the mother of Walter, who gave this name to his second daughter. Wightman suggests that Walter was the younger brother, because he arrived in England later than Ilbert “in the train of William FitzOsbern” and on the assumption that Ilbert was named by their mother because he was her older son and therefore held a position of greater importance in Normandy. Orderic Vitalis records that King William granted “insulam Vectam et comitatum Herfordensem” to “Willelmo dapifero Normanniæ Osberni filio”, and sent him “cum Gualterio de Laceio” to fight “contra Britones” [Welsh], during which they first invaded “Brachaniaunos” and defeated “Guallorum reges Risen et Caducan ac Mariadoth”, dated to 1071. The Historia sancti Petri Gloucestriæ records that "Walterus de Lacy" donated “terras de Ledene" in 1080, with the consent of "rege Willelmo seniore", when "Waltero filio suo tunc parvulo" entered the monastery, "tempore Serlonis abbatis" [abbot from 1072 to 1104].
The Historia sancti Petri Gloucestriæ records the death "VI Kal Apr" 1085 of "Walterus fundator Sancti Petri Hereford", his burial "Gloucestriæ in capitulo", and the donation of "villam quinque hidarum Duntesborne tempore Serlonis abbatis" [abbot from 1072 to 1104] made by "uxor eius Ermelina". 1