These structures belong to a category of prehistoric and early medieval defended enclosure in which a natural peninsula does much of the defensive work; a bank, ditch, or stone rampart was thrown across the neck of the headland, and the sea on three sides took care of the rest. The result was a stronghold that required relatively modest effort to make formidable, and the Irish coastline is studded with them, though most go unnoticed by anyone not specifically looking.

Calf Island Middle sits in Roaringwater Bay, an inlet already crowded with islands large and small, and the presence of a promontory fort there points to a long history of human activity in this archipelago. The islands of Roaringwater Bay were inhabited and strategically significant well before the medieval period, and coastal forts of this type are generally associated with the Iron Age or the early centuries of the first millennium, though precise dating is difficult without excavation. Who built this particular fort, and what community it served or protected, remains unknown. The island is small and largely uninhabited today, which means the earthworks have likely been spared the kind of agricultural disturbance that has erased similar sites on the mainland.