Known locally as Dúinín, the little fort, it is a place that has technically been catalogued but, by the accounts available, never actually visited by anyone recording it formally. What survives is the ghost of a promontory fortification, the kind once constructed by cutting a defensive bank and fosse across the narrow neck of a headland to create a naturally sheltered stronghold on the seaward side. The fosse is a ditch, dug to reinforce the bank in front of it, and together they would have made the landward approach genuinely difficult. At Dúinín, the promontory itself has since been severed entirely from the island, leaving a strip of land roughly 55 metres long and 10 metres wide, isolated by the sea that gradually claimed the southern connection to Inishshark.

The earthworks that once defined this place have been largely destroyed, though traces persist. A bank measuring more than nine metres in length and around four and a half metres wide survives at the southern end of the island, accompanied by the remains of an external fosse of similar width. Along the western edge, a lower and narrower bank runs for more than twenty-four metres, standing only about sixty centimetres above the surrounding ground. T. J. Westropp, the prolific antiquarian who documented so many of the west of Ireland's overlooked monuments, noted the site in 1914, and his reference remains one of the few anchors for what little is known about it. Inishshark itself was permanently evacuated in 1960, when its small community was resettled on the mainland, so the island and its offshore fragment have been largely left to their own slow erosion since.