It is the remains of a fulacht fia, a type of prehistoric cooking or processing site found in considerable numbers across Ireland, typically Bronze Age in origin. The characteristic signature of these sites is a spread of fire-cracked stone, the debris from a method of heating water by dropping superheated rocks into a trough or pit. Over time, as stones shatter and become unusable, they accumulate in a distinctive horseshoe-shaped or irregular mound around the working area.

The spread at Knocknabehy measures roughly 18.5 metres north to south and 11 metres east to west, an irregularly shaped patch of burnt material sitting on an east-facing slope. Running east to west directly through the centre of the deposit is a stream, now piped underground. The proximity to running water is not coincidental. Fulachtaí fia are almost invariably found beside streams, springs, or other reliable water sources, since access to water was fundamental to how the site functioned. The stream here, though no longer visible at the surface, once defined the logic of the location entirely.