That collision of eras is one of the more quietly telling things about the site at Ballynahown, where the remnants of a large cashel, a stone-walled ringfort of early medieval type, sit on slightly raised ground in a flat, rocky landscape thick with scrub. To the west, the ocean is visible through gaps in the vegetation; in almost every other direction, the site has largely swallowed itself.

The antiquarian Thomas Johnson Westropp visited and recorded the fort in 1905, naming it Cahermoyle and noting that it was the largest of the forts in the surrounding area, with internal dimensions of around 45.72 metres. His assessment was not encouraging: he described the cashel as crossed by a long boundary wall and so entirely overthrown as to be indescribable. A century on, that judgement still broadly holds. The roughly circular enclosure measures approximately 45 metres north to south and 37 metres east to west, and what survives of the defining bank of stone and earth is unevenly preserved. The best-preserved stretches run from the south-south-east around to the west, where the bank reaches widths of up to 4.5 metres and outer facing-stones are still visible rising to around half a metre or more. Elsewhere, particularly from the west around to the north-north-east, the bank appears to have been largely removed or robbed out. The interior is uneven and rises noticeably above the surrounding ground level on the south-western side. The cashel does not stand alone; two further cashels lie within roughly 100 metres of it, one to the south-south-west and another to the north-west, all of them set within an extensive ancient field system that spreads across Knockaunsmountain. The site appears on the 1915 Ordnance Survey six-inch map, marked with hachures indicating an earthwork, though by 1996 it was classified only as an enclosure, a category that tends to reflect how much a site has lost to time and reuse.