O'Sullivan was part of a confederation of Gaelic leaders who had gone into rebellion against Elizabeth I of England and were aided by King Philip III of Spain who sent troops under Don Juan del Águila.
O'Sullivan was part of a confederation of Gaelic leaders who had gone into rebellion against Elizabeth I of England. He was aided by King Philip III of Spain, who sent an invasion force to Kinsale under the command of Don Juan del Águila.