It measures roughly fifteen metres along its longer axis and ten metres across, and it is composed almost entirely of burnt and fire-cracked stone. That description, modest as it sounds, identifies it as a fulacht fia, one of the most common prehistoric monument types in Ireland and one of the least understood.

A fulacht fia is essentially the debris left behind by a Bronze Age cooking method. The basic principle involved heating stones in a fire and then dropping them into a water-filled trough to bring the water to a boil, a technique repeated until the stones shattered from thermal stress. The cracked, blackened fragments were raked aside after each use, and over time this process built up a low horseshoe-shaped or oval mound around the trough. The presence of a stream nearby is typical; these sites are almost always found close to running or standing water, which was needed in quantity. At Derryorgan, the stream sits to the northwest of the mound, and the burnt material is exposed in cross-section where a drain has cut through the northwest edge, offering an accidental but revealing glimpse into the depth of accumulated waste beneath the surface. Whether the site was used for cooking, bathing, industrial processes, or some combination of these remains a live question in Irish archaeology; the fulacht fia has resisted tidy interpretation for decades.