King Henry II of England appointed him Governor of Limerick and granted him vast estates in Leinster and Munster. De Burgh's castles at Tibberaghny (County Kilkenny), Kilsheelan, Ardpatrick and Kilfeacle were used to protect King John's northern borders of Waterford and Lismore and his castles at Carrigogunnell and Castleconnell were used to protect Limerick. He was Seneschal of Munster (Royal Governor) from 1201 to 1203.
Sometime in the 1190s, William allied with the King of Thomond, either Domnall Mór Ua Briain, King of Thomond (died 1194) or his son Murtogh, and married one of his daughters. This alliance probably took place during the reign of Murtough, as up to the time of his death Donal 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.
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 (reigned 1190–1224) 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.
The Annals of the Four Masters recorded his passing thus:
The identity of William's wife is uncertain. A late medieval genealogy 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. A book of genealogies recorded in the 15th century by Cú Choigcríche Ó Cléirigh, one of the Four Masters (published in Annalecta Hibernica 18), indicates that the mother of Richard Mor de Burgh, William's son and successor, was the "daughter of the Saxon [Angevin] king", an illegitimate daughter of Henry II of England via~ the house of Normandy. Such a connection would explain the use of the term consanguineus kinsman by Edward I of England to describe Richard Óg de Burgh, 2nd Earl of Ulster.
William had three known children (with the spelling Connaught being used in titles of English nobility):
Walter de Burgh of Burgh Castle, Norfolk. =Alice | |_____________________________________________________________________________________________________________________ | | | | | | | | William de Burgh, died 1205. Hubert de Burgh, 1st Earl of Kent, died 1243. Geoffrey de Burgh, died 1228. Thomas de Burgh | (issue; John and Hubert) |_________________________________________________________________________________________________________ | | | | | | Richard Mór de Burgh, 1st Baron of Connaught Hubert de Burgh, Bishop of Limerick, died 1250. Richard Óge de Burgh | | | ____________________________________________________________| de Burgh Earl of Ulster, | | | Burke of Castleconnell, County Limerick | | | Mac William Iochtar Bourke of County Mayo. Hubert William Richard | | | | | |_________________ Clan Mac Hubert? Richard an Fhorbhair | | | | | _______________________________________________________________| Sir David Donn Sir William Ruad | | | | died 1327. | | | Clan Mac David Ulick Burke of Annaghkeen, died 1343. Raymond Walter Óge | | Richard Óg Burke, died 1387. | | Burke of Clanricarde