William de Barry
William's father, Philip de Barry, came to Ireland in 1185 to assist his uncle Robert Fitz-Stephen, and his first cousin Raymond Fitz-William -Fitz-Gerald (also known as Raymond Le Gros), in their efforts to recover the cantreds of Killede, Olethan and Muscarydonegan that had been expropriated by another (half) first cousin, Ralph Fitz-Stephen (died 1182), the grandson of Nesta by Stephen, Constable of Cardigan. Robert Fitz-Stephen eventually ceded these territories to Philip de Barry, his half-nephew.
On 24 February 1206, King John of England confirmed William de Barry, Philip's son, in the possession of these territories and, by letters patent, conferred on him the Lordships of Castlelyons, Buttevant and Barry's Court in East Cork. The family would eventually acquire the honours of Viscount Buttevant and Earl of Barrymore.