In 1589 Maguire succeeded his father and inherited lands in Fermanagh with a retreat in the islands of Lough Erne which he considered impregnable. He retorted to Fitzwilliam's demand that he allow the queen's writ to run in his country: ""Your sheriff shall be welcome but let me know his eric honour price that if my people should cut his head off I may levy it upon the country"". His argument was that he had already paid 300 beeves to Fitzwilliam to keep the sheriff out. Nevertheless, Captain Willis was made sheriff in command of 100 men, and disaffected members of the clan were encouraged to defy Maguire. In 1590 Maguire drove the sheriff and his men into a church and besieged them there, whereupon Tyrone intervened to save the besieged from death. Fitzwilliam then invaded the country, proclaimed Maguire a traitor, and took Enniskillen.