It is a fulacht fia, the term given to a type of prehistoric cooking site found in great numbers across Ireland, typically consisting of a trough, a nearby water source, and a surrounding mound of fire-cracked stone built up through repeated use. Whoever worked here would heat stones in a fire, drop them into a water-filled trough, and use the resulting heat to cook meat or process other materials. The process sounds improvised, but the accumulated debris, dense concentrations of shattered stone in charcoal-blackened soil, speaks to sustained, repeated activity over time.

The mound at Derryherbert is well preserved and reasonably substantial: roughly 10.5 metres north to south and 7.5 metres east to west, with a central depression around two by three metres that opens towards the stream on its eastern side. The asymmetry of the mound reflects the natural fall of the ground; the south side rises to about 1.25 metres, while the north and west arcs taper away to less than half a metre, merging almost seamlessly with the surrounding pasture. Along the eastern edge, where the mound meets the stream, several large stones are set at the base, possibly placed deliberately to stop the accumulated burnt material slipping into the water. A single flat stone, roughly 1.25 metres long, protrudes from the south-western edge of the mound. Just to the north, large flat slabs of natural bedrock line the stream bank, which may have served as a practical working surface. The stream itself flows south to join the Owenmore River, and on a clear day Croagh Patrick is visible on the far horizon to the north-north-west.