After the fall of Kinsale in 1601, O'Sullivan remained one of the few Irish leaders in the region who continued to fight. In early 1602, with English forces tightening their grip, O'Sullivan went north to Ulster to consult with O'Neill. In June, the English attacked Dunboy Castle, one of the more prominent forts in O'Sullivan territory. While O'Sullivan had returned south from Ulster, he was not present at the siege.