 Unlike his father and his uncle Thomas Luttrell, who both went on to become eminent judges, he did not practice at the Irish Bar.Nor was he a Bencher of the King's Inn, which his father had helped to found, although he was a party to the renewal of the lease of the Inn from the Crown in 1567.He may have had some legal training, since Thomas Butler, 10th Earl of Ormonde, of whom he was a close associate, appointed Barnewall in 1556 as steward and receiver of all the Earl's lands within the Pale.