In doing so, it cuts the entire tip of the island clean off from the rest of it, sea to sea. That kind of feature has a name and a purpose: a promontory fort, a type of enclosure found along Ireland's Atlantic coastline in which a headland or peninsula is defended not by encircling it entirely but by throwing up a barrier across the neck of land that connects it to the mainland, or in this case, to the rest of the island.

The bank itself is built of earth with a facing of drystone revetment, a technique in which stones are laid without mortar to stabilise and retain an earthen core, and comes to roughly four metres in width. Running along its top is a later stone wall, probably added in the nineteenth century, though the underlying bank is considered likely to be of considerably greater antiquity. On the south-western side there are traces of a fosse, a defensive ditch that would have deepened the obstacle presented by the bank, and within the thickness of the bank itself there are intermittent remains of what may be hut structures. To the south-west of the whole arrangement lies a field system, suggesting that whatever community used this end of the island also worked the land there. The site was noted by archaeologist M. Gibbons as a possible ancient rampart. Inishshark, an uninhabited island off the Connemara coast evacuated by its last permanent residents in 1960, preserves this kind of layered landscape precisely because so little has disturbed it since.