This Manus, succeeded his cousin as chief, Manus O'Cathain had a difficult period in which to rule, due to the bitter struggle with the MacQuillans. In 1542 the O'Cathain Sept with the aid of the O'Donnells made a major effort to destroy the McQuillans, with the destruction of much property and immense herds of cattle captured. A counter raid by the McQuillans and MacDonnells of Antrim into the country of O'Cathain, saw the driving off of cattle. Manus O'cathain with the aid of the Sweeneys overtook the MacQuillans and defeated them.
In the year 1542, the McQuillans with the support of the English attacked Manus O'Cathain Castle at Limavady and slew all the warriors who were in the town. At the close of the same year, Manus O'Cathain made his submission to the King of England and signed an indenture of peace. Manus O'cathain was killed in 1548, taking part in a quarrel among the O'Donnells, at a strath along the banks of the Finn near Ballybofey