The Oxford Dictionary of National Biography states, [1]
In 1270 a major conflict arose with Aodh Ó Conchobhair, who had succeeded his father in 1265. During negotiations the earl [Walter de Burgh] surrendered his brother, William, to Aodh as a pledge. When he heard that William had been imprisoned, he is said to have slain Toirdhealbhaigh Ó Briain, who was allied with the Ó Conchobhair, by his own hand. Aodh then defeated the earl's army at Áth an Chip, near Carrick-on-Shannon, and executed William.
[1] Robin Frame, "Burgh, Walter de, first earl of Ulster (d. 1271)," Oxford Dictionary of National Biography (Oxford University Press, 2004, online edition), [Oxford_Dictionary_National_Biography], [OxfordDNB(UM)].