Under Henry VIII's 'surrender and re-grant' scheme, the O'Brien's were granted the title 'Earls of Thomond' and they agreed to profess loyalty to the King of England.
As part of the conditions of this his family abandoned Irish traditions and renounced the Catholic faith, becoming adherents of the new Anglican church.
They ruled the territory of North Munster and lived in great splendor. The castle was surrounded by beautiful gardens and it was reputed to have a herd of 3,000 deer. The reign of the O'Briens came to an end with the arrival of the Cromwellian troops and the castle and its grounds were surrendered. The O'Briens never returned to Bunratty but later they built a beautiful residence at Dromoland Castle, now a luxury 5 star hotel.