These forts work by identifying a headland or projecting spur of land where the sea does most of the work: cliffs and water guard three sides, and a constructed barrier, typically a bank or stone wall cutting across the neck of the promontory, secures the fourth. The result is an enclosure that required relatively little effort to render formidable, which may explain why they were built at all along coasts where the land itself offers so little flat ground.

Promontory forts are found all around the Irish coast, and while many date to the Iron Age, occupation at individual sites can be difficult to pin down without excavation. Inishodriscol, also known as Sherkin Island's neighbour in Roaringwater Bay, sits in a stretch of water that was well travelled in early medieval times, when the O'Driscoll family, one of the dominant maritime dynasties of Munster, controlled the sea-lanes and island territories of this part of Cork. The name Inishodriscol reflects that association directly, the island's identity bound up with the same family whose castles and influence dotted the surrounding coastline for centuries. Whether the fort predates that period or was in some way connected to it remains the kind of question that only careful fieldwork could begin to answer.