According to the Annals of Inisfallen, in 1201, William and the sons of Domnall Mór led a major joint military expedition into Desmond, slaying Amlaíb Ua Donnabáin among others.

[4] In 1200, "Cathal Crobhdearg Ua Conchobair went into Munster, to the son of Mac Carthy and William de Burgh to solicit their aid."

Cathal Crobhdearg Ua Conchobair (King of Connacht, 1190–1224) faced much opposition (mainly from within his own family) and wished to engage de Burgh's aid to further secure his position.

Here, William de Burgh (and the sons of Rory O'Flaherty) conspired to kill Ua Conchobair but the plot was foiled (apparently by holy oaths they were made to swear by the local Coarb family).

[6] The Annals of the Four Masters recorded his passing thus: "William Burke plundered Connacht, as well churches as territories; but God and the saints took vengeance on him for that; for he died of a singular disease, too shameful to be described."

[7] A late medieval genealogy found in the Book of Lecan (dated c.1397-1418) records his marriage to an unnamed daughter of Donmal Mor mac Turlough O'Brien,[8] and the descent of the Earls of Ulster and Clanricarde from their son Richard.