Sometime in the 1190s, William allied himself with the King of Thomond, either Domnall Mór Ó Briain, King of Thomond or his son Muirchertach. In 1193, William de Burgh married the daughter of Domnall Mór Ó Briain, King of Thomond. This alliance probably took place during the reign of Muirchertach, as up to the time of his death Domnall Mór had been at war with the Normans. At any rate no more wars are recorded between the two sides for the rest of the decade. 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. From 1199 to 1202 de Burgh led military campaigns in Desmond with the aid of the Ó Briain. Success in the west and south allowed de Burgh to conquer the Kingdom of Connacht, which although he had been granted probably before 1195, he had never occupied. Cathal Crobhdearg Ua Conchobair, King of Connacht, fought a successful counter-attack against the Anglo-Norman castles in Munster, including de Burgh's castle of Castleconnell. Further fighting led to loss of three castles and property, all of which was eventually retrieved with the exception of much of Connacht.
Connacht
In 1200, "Cathal Crobhdearg Ua Conchobair went into Munster, to the son of Mac Carthy and William de Burgh to solicit their aid." This marked the start of de Burgh's interest in the province. King Cathal Crobderg Ua Conchobair faced much opposition, mainly from within his own family and wished to engage de Burgh's aid to help secure his position. The following year William and Ua Conchobair led an army from Limerick to Tuam and finally to Boyle. Ua Conchobair's rival, Cathal Carragh Ua Conchobair marched at the head of his army to give them battle but was killed in a combined Burke/Ua Conchobair onslaught after a week of skirmishing between the two sides. William and Ua Conchobair then travelled to Iar Connacht and stayed at Cong for Easter. Here, William 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. However, when de Burgh demanded payment for himself and his retinue, battle finally broke out with over seven hundred of de Burgh's followers said to have been killed. William, however, managed to return to Limerick. The following year in 1202, William returned and took revenge for his army that was destroyed a year early. He took the title “Lord of Connacht” in 1203.
Death
He died in winter 1205/1206 and was interred at the Augustinian Priory of Athassel in Golden which he had founded c. 1200. 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."
Family
William's wife was the daughter of Domnall Mór Ó Briain, King of Thomond, whom he married in 1193. A late medieval genealogy found in the Book of Lecan records his marriage to an unnamed daughter of Donmal Mor mac Turlough O'Brien, and the descent of the Earls of Ulster and Clanricarde from their son Richard. William had three sons and at least one daughter by his wife, the daughter of Domnall Mór Ó Briain, King of Thomond :
