It is a fulacht fia, a type of prehistoric cooking site found in great numbers across Ireland, typically consisting of a horseshoe-shaped mound built up from the accumulated debris of repeated use: stones that were heated in a fire and then dropped into a water-filled trough to bring it to the boil, shattering in the process. The mound at Derrymihin measures roughly five metres north to south and four metres east to west, rising only about thirty centimetres above the surrounding ground. Its opening, nearly two metres wide, faces west, and the dark, charcoal-enriched soil mixed through the heap of heat-fractured stones is the residue of what may have been dozens or hundreds of individual episodes of use over a long period in prehistory.

What makes this particular site quietly interesting is not so much what it is, but where it sits in relation to its neighbours. Two further fulachtaí fia lie within roughly sixty metres, one to the northwest and one to the west, making this a small cluster of monuments on the same boggy hillside. Whether they represent repeated use of a favoured location across generations, or more or less contemporary activity by a community that needed multiple cooking sites, is the kind of question the physical remains alone cannot answer. What is clear is that the western edge of the mound at Derrymihin is being eroded by farm machinery, a slow attrition that is not unusual for low earthwork monuments in working agricultural landscapes.