This cashel, located at An Chill Bheag, consists of an almost circular enclosure measuring 18.5 metres in diameter from east to west. Though time has taken its toll on the structure, with its stone walls now completely collapsed and partly covered by grass, the site still commands an impressive position above the bay, surrounded by good quality pasture land that has likely been grazed for centuries.

The flat interior of the fort suggests it once provided a level platform for buildings, perhaps housing a farming family and their livestock during troubled times. Ringforts like this one, built with dry stone walls rather than earthen banks, are known as cashels and are particularly common in the rocky landscapes of western Ireland. They typically date from the early medieval period, roughly between 500 and 1200 CE, when they served as defended farmsteads for prosperous families.

This particular site was documented as part of the Archaeological Survey of County Donegal, a comprehensive catalogue compiled by Brian Lacey and his team in 1983. The survey recorded field antiquities across the county spanning from the Mesolithic period right through to the 17th century, helping to preserve knowledge of sites like Dun Ailinn that might otherwise fade from memory as the physical remains continue to deteriorate in the Irish weather.