An Dún Beag, as it is known in Irish, is a site in the process of disappearing. In late January 2014, a substantial portion of the fort's western edge collapsed into the water during storm damage, continuing an erosion that antiquarians had already noted in the early nineteenth century. By the time excavations began in 1977, initiated by the Office of Public Works in an effort to record the site before further loss, the inner stone rampart stretched only 29 metres across the promontory, roughly half the length that the artist and geologist George Victor Du Noyer had measured in 1856.

What survives is still a remarkably layered piece of early medieval engineering. The defences consist of four earthen banks, five fosses (rock-cut ditches), and an inner drystone rampart up to 6.35 metres thick and just over three metres high, with a carefully designed entrance passage flanked by two guard-chambers built into the wall itself. Bolt holes in the passage walls allowed a wooden bar to be operated from within the chambers, giving those inside precise control over who could enter. Running beneath the causeway is a souterrain, a type of underground stone-built passage associated with early Irish defended sites, extending 16.5 metres and narrowing abruptly at a dogleg turn marked by two large upright stones. In the interior, a stone-flagged path leads to a clochaun, a dry-stone corbelled structure circular on the outside and rectangular within, its internal diameter measuring 7.5 metres. Excavation of the clochaun revealed two distinct occupation layers, with hearths, stake holes, and animal bones suggesting intermittent habitation, perhaps in periods of emergency rather than continuous settlement. Bones of pig, sheep, goat, cattle, deer, birds, and fish were recovered across the two phases. The excavator, Barry, argued the clochaun's span was likely too wide to have supported a full corbelled roof and proposed instead that lean-to shelters of wattle and wood were erected against its inner walls.

Radiocarbon dating places the earliest activity on the site considerably earlier than the medieval occupation. A charcoal layer beneath the rampart produced a date of around 580 BC, pointing to Iron Age or earlier use of the promontory. The inner fosse dates to around 800 AD, and the clochaun occupation layers to the tenth and eleventh centuries. The 1890s saw reconstruction work on the site, and it is possible that some of what visitors see today reflects the judgements, and perhaps the misreadings, of the workmen involved at that time, who may have incorporated the remnants of later field walls into what they took to be the original rampart line.