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.