What remains is a roughly rectangular area, approximately 30 metres by 20 metres, where the cliff face on the east and south sides does the work of a wall, and a shallow curving fosse, flat-bottomed and around six metres wide, traces a line along the west and north. No bank survives above the fosse, no trace of an entrance, no clear evidence of what once stood here beyond the cut in the ground itself.

Promontory forts, sometimes called cliff castles, are a class of coastal defended site found widely along the Atlantic seaboard of Ireland and Britain. The typical arrangement uses natural cliff edges on the exposed sides and one or more earthen banks and ditches, a fosse being the ditch component of such a defence, to close off access from the landward side. Most date to the Iron Age, though some were used across very long periods. The example on High Island is tentative enough that the word "possible" attaches to every description of it. The fosse survives; the bank that would once have sat alongside it has gone entirely, whether quarried, eroded, or simply never substantial enough to outlast the Atlantic weather. High Island itself is known better for its early Christian monastery, whose remains include a small oratory and cells, so this potential fortification on the opposite end of the island occupies a much quieter place in the archaeological record, noted but unconfirmed.

High Island is uninhabited and access is by boat from the Connemara mainland, dependent on sea conditions that can make landing difficult or impossible for considerable stretches of the year. The eastern headland where this feature sits is at the far end of an already remote and exposed place, and the site itself has not been subject to formal inspection. What a visitor would find is a grassy clifftop, a shallow depression in the ground, and the possibility, never fully resolved, of something deliberately made.