The headland on which it once stood juts out above the sea stacks and natural arches of Carrignaweelaun Bay, and the Atlantic has been steadily reclaiming the interior for long enough that only a thin strip of the original structure survives. What makes the site quietly striking is precisely this incompleteness; you are looking at the edge of something that was already, in its day, designed around an edge.

A promontory fort is one of the more elemental forms of early Irish defensive architecture, using the natural shape of a coastal headland as its primary barrier and blocking off the landward approach with earthen banks and a fosse, the term for a ditch cut to heighten the effective height of the bank above it. At Stack's fort, there were originally two such banks with a fosse between them. The outer bank, now heavily disturbed by later field fences built across it, ran for around eleven metres and stood three metres above the fosse, which was itself two and a half metres wide and fifteen metres long. The inner bank reached a maximum of two metres over the fosse on its interior side. These measurements come from C. Toal's North Kerry Archaeological Survey, published in 1995, by which point the erosion was already severe. The fort sits in a landscape that is unusually dense with related monuments; the Crescent Fort lies to the north, and to the south another promontory fort and Pookeenee Castle are both visible from roughly the same vantage point, suggesting this stretch of coastline was a place people returned to, generation after generation, for reasons of defence or occupation or simply the logic of geography.