The site began as a fulacht fia, the Irish term for a type of prehistoric cooking or processing site found in enormous numbers across Ireland, typically identified by a horseshoe-shaped mound of heat-shattered stones left over from repeated cycles of heating rocks and dropping them into water-filled troughs. At Fermoyle, that familiar horseshoe form survives clearly, open at its eastern end, and the mound extends some 14.8 metres along its north-south axis, rising between 0.6 and 1 metre above the surrounding ground. A small stream runs close by to the south-east, which would have provided the water supply essential to the site's original function.

What makes Fermoyle quietly unusual is what happened to the mound long after its prehistoric use had ended. At some point it was repurposed as a ceallúnach, a type of informal or unconsecrated burial ground, often used in Ireland for unbaptised children or others excluded from consecrated church cemeteries. The eastern opening of the horseshoe now contains two upright stone slabs and several quartz blocks, features that appear to belong to this later burial phase rather than to the prehistoric cooking activity. The combination is not entirely without parallel in Ireland, where ancient earthworks were sometimes pressed into secondary use, but the specific pairing of a fulacht fia with a ceallúnach gives this particular mound an unusually layered human significance. The commanding position above the valley would have made the site a visible landmark across successive generations, which may partly explain why it kept attracting use.