When news arrived in summer 1580 that James Eustace, third viscount Baltinglass, had entered into rebellion with Gaelic elements in Leinster, Tudor officials in Ireland could barely contain their mystification. How, they wondered, had this important Tudor noble, whose family had for centuries served as a bulwark against the menace of marauding Gaelic clans, become ‘so forgetful of himself’? In seeking to explain the rebellion historians too have sought to understand Eustace's motivation, and in so doing have come to emphasize his integral position in conceiving and leading the revolt. Yet this approach has obscured the rebellion's Gaelic dimension. The rebellion would not have been possible without the support of the O'Byrnes and O'Tooles who sought to uproot the English county administration then being erected in place of their quasi-independent lordships. Fiach MacHugh O'Byrne emerged from this interest to become the rebellion's true strength and in August 1580 dealt a shattering defeat to the English army in Ireland.