It stretches forty-five metres across the neck of a south-facing headland, several metres thick, and it is the only thing marking this spot as something other than rough coastal grazing land. No Ordnance Survey map has ever recorded it. Until relatively recently, no published writer had noted it at all.

The structure is a promontory fort, a type of enclosure common along the Irish coastline in which a defensive wall or bank is thrown across the narrow approach to a headland, letting the sea cliffs do the remaining work. The headland here measures roughly 110 metres in length and 45 metres across, with outcropping rock visible on its interior surface. The surrounding island bears the marks of former cultivation in the form of lazybeds, the low parallel ridges left behind by a technique of raised-bed agriculture once widespread across the west of Ireland, suggesting that Duvillaun supported a human population long after whoever built the fort had gone. The site came to wider attention through a 1999 unpublished MA thesis by Markus Casey at the National University of Ireland, Galway, which surveyed coastal promontory forts across counties Sligo, Mayo, Galway, and Clare. Notably, even Casey's team did not set foot on the headland; the fort was identified entirely from aerial observation, which gives it an additional layer of remoteness, known to scholarship but never physically documented at ground level.