It rises only twenty centimetres above the surrounding ground, measuring roughly twelve and a half metres north to south and nearly fifteen metres east to west. Easy to walk past without a second glance, it is in fact the accumulated residue of prehistoric cooking activity, its dark, humped profile built up entirely from burnt stone and charcoal-stained soil deposited over many generations of use.

This type of site is known as a fulacht fia, a term used in early Irish literature and now applied by archaeologists to the thousands of burnt mound sites found across Ireland and Britain. The working principle, broadly consistent across such sites, involved heating stones in a fire and then dropping them into a water-filled trough to bring the water to a boil. The stones fractured and blackened with repeated use and were discarded at the edge of the trough, gradually building up the characteristic horseshoe or D-shaped mound visible today. The precise purposes served by these sites remain debated; cooking is the most widely accepted explanation, though brewing, hide-working, and bathing have all been proposed by researchers. At Ardglass, the southern arc of the mound has been partially cut away by the drain running alongside it, which both exposes the burnt material within and hints at the typical pattern of such sites being positioned near reliable water sources.