It measures roughly ten metres by five, rises to about a metre in height, and opens to the west. To a passing eye it might read as a natural rise in the ground, an unremarkable ripple in the landscape. It is, in fact, a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish archaeological record.

Fulachtaí fia are burnt mound sites, found in their thousands across Ireland and dating mostly to the Bronze Age, though some continued in use into the early medieval period. The typical form is a horseshoe or crescent of scorched, heat-shattered stone, accumulated over repeated use of a nearby trough or pit into which water was heated by dropping fire-cracked rocks. What exactly that water was used for remains genuinely contested. Cooking is the most widely accepted explanation, but brewing, textile processing, and bathing have all been seriously proposed by archaeologists. The characteristic crescent shape at Ballynabrocky, opening to the west, fits the classic profile: the discarded burnt stone piling up on either side of the working area over what may have been many generations of use. The proximity to a running stream is equally typical, since a reliable water source was essential to the whole operation, whatever that operation actually was.