William arrived in Ireland from England sometime around 1185, after he had been given large tracts of land by King John. It wasn't long after his arrival that he took the office of Governor of Limerick and was given the manors of Ardpatrick and Kilsheelan.

In 1200 A.D. William was granted the castle of Tibraghty in Co. Kilkenny.

William married a daughter of Donal Mor O'Brien, King of Thormond which greatly increased his power in the area and in 1200 he set his sights on Connaught.

After many battles and campaigns sometimes on the side of the O'Connors - Kings of Connaught and sometimes against them, He eventually joined forces with Cathal Crov Dearg and in a skirmish near Boyle, Co Roscommon his main rival for the province, Cathal Caragh was slain.

After this skirmish both William and Cathal Crov Deargh went to the Monastery at Cong where they spent Easter.

During their stay at Cong, William billeted his soldiers with various clans around the province but a false rumour was spread that William was dead; at this news, all 900 of William's soldiers were put to the sword by their hosts.

William returned to Limerick to form a new army and the following year he marched into Connaught with the sole purpose of avenging the deaths of his murdered soldiers.

On the banks of the Shannon at Meelick, he built a castle and from there, laid waste the province, plundering the monasteries of Clonfert, Knockmoy, Mayo, Clonmacnoise and Cong, burning, killing and pillaging as they went.

His former ally Cathal Grog Dearg complained bitterly to King John and William was recalled to England where he surrended his castles to the King. The complaint was taken seriously by the King and a commision was set up to investigate, but the matter was abandoned, due no doubt to the influence of Williams brother, Hubert.

William was eventually given all his castles and land back except for Connaught, which was to remain the King's territory.

William returned to Ireland in 1204, and when he died in 1205/6 he was buried in Athassel Abbey which he had founded 5 years earlier.

William had a son, Robert de Burgh who with his army eventually over-ran Connaught and parcelled up the lands between themselves finally ending the power of the O'Connors and starting the spread of one of Irelands largest families.