This Rory, son of Manus and grandson of Donough. succeeded his cousin as chief. Rory attended the conciliation Parliament in Dublin in 1585. A few years later he gave refuge to 200 of the Queen's troops. These troops had come to Tyrone to stiffen resistance against the O'Donnells. The O'Donnells laid seige to O'Cathain's castle at limavady on the Roe river. However Rory sent a message to O'Donnell and reminded him that he was his foster-father, and that he had at all times been friendly with the O'Donnell family. O'Donnell forces withdrew back to his own country. The Annuals of Ulster record the death of Rory O'Cathain in April 1598, and that his successor was his son Donnell Ballagh O'Cathain.
Walter Devereaux, Earl of Essex, in 1573 wrote:
"The strength of this man (Rory O'Cathain) is his own nation, who are able to make 140 horsemen and about 400 foot".