The island belonged to the O'Flahertys until 1380, when the O'Malleys captured it. In the 16th century, according to local tradition, a Spanish pirate or Barbary corsair named Alonzo Bosco built a stronghold on Port Island, where the Cromwellian fort stands today. According to the tales he raided the Irish coast and shipping in the area. "Don" Bosco was supposed to have been an ally of Gráinne O'Malley, chieftain of the O'Malley clan and 'Ireland's pirate queen'. One story had them stretching a defensive iron chain across Inishbofin's harbour entrance to make it impassable to enemy ships. Across the harbour entrance from Port Island sits 'Dún Gráinne', the fort where Gráinne O'Malley supposedly lived.