A member of the famous O'Neill dynasty, Sir Felim was the grandson of Sir Henry Og MacShane O'Neill and great-grandson of Henry MacShane O'Neill, one of the O'Neills who remained in Ulster after the Flight of the Earls despite the difficulties brought on by land confiscation and the Plantation of Ulster. O'Neill was a member of the Irish Parliament in the 1630s and trained as a lawyer at King's Inns in London. His Father was Sir Turlough MacShane O'Neill and His mother was Catherine Ny.