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.
Read more about this topic: William De Burgh