The feature is a fulacht fia, a type of prehistoric cooking site found in great numbers across Ireland, particularly in low-lying, wet ground close to running water. They are typically identified by their characteristic mounds of fire-cracked stone, the discarded debris of repeated heating.

The mechanics of a fulacht fia are straightforward but effective. Stones were heated in a fire, then dropped into a water-filled trough, bringing the water rapidly to the boil. Food, most likely meat wrapped in straw or hide, could then be cooked in the trough. The Derreen example measures sixteen metres north to south and twenty metres east to west, a spread of burnt material substantial enough to suggest prolonged or repeated use over time. It lies on the northern side of a stream, and the ground around the spread remains waterlogged, a condition that likely helped preserve the deposit and that mirrors the wet, marginal land where the vast majority of these sites are found across the Irish landscape. The site was recorded as being in tillage, meaning it sits within cultivated farmland, which is not unusual; many fulachtaí fia survive as low, inconspicuous mounds only partially visible at the surface.