It measures roughly eleven and a half metres north to south, nine metres east to west, and rises less than a metre from the ground. Beneath a thin layer of topsoil lies a mass of burnt material, the accumulated residue of a fulacht fia, one of the most common yet least understood monument types in the Irish archaeological record.

A fulacht fia is a prehistoric cooking site, typically Bronze Age in date, built around a trough filled with water that was heated by dropping fire-cracked stones into it. The stones, once used, were discarded in a horseshoe-shaped mound around the trough, and it is precisely this mound of shattered, blackened stone that survives at Lisleagh. The proximity to a stream is no coincidence; access to a reliable water source was a practical requirement, and fulachtaí fia are found near streams, rivers, and boggy ground throughout Ireland in considerable numbers. What is less certain is everything else: the exact purpose, the social context, the people who used them. Brewing, hide-working, and communal feasting have all been proposed alongside the straightforward explanation of cooking, and the debate remains open.