Brian MacArt O'Neill was a member of the O'Neill Dynasty, the leading Gaelic family of Ulster. He was the son and heir of Art MacBaron O'Neill of Oneilland and a grandson of Matthew O'Neill, 1st Baron Dungannon. He may have taken part in Tyrone's Rebellion, a conflict in which several of his brothers were killed. Along with the rest of his family he was pardoned as part of the Treaty of Mellifont in 1603. Three of his brothers served as mercenaries in the Spanish Army. The best-known of them was Owen Roe O'Neill.