Roughly seven metres across and barely half a metre high, it would be easy to read as a natural irregularity in the landscape. It is not. The mound is composed of burnt stone and charcoal, the distinctive debris left behind by a fulacht fia, a type of prehistoric cooking or industrial site found widely across Ireland, typically beside a water source or in marshy ground.

Fulachtaí fia, the plural form, are among the most common archaeological monument types on the island, yet they remain poorly understood in terms of their precise function. The standard interpretation holds that stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil, with the cracked and shattered stones discarded into a characteristic horseshoe-shaped mound nearby. The boggy terrain at Knockreagh fits this pattern well; such sites tend to cluster where water was reliably close to the surface. The ground here has since been drained and brought into agricultural use, and a field drain running roughly northeast to southwest passes about five metres to the north of the mound, a reminder that managing water in this landscape has been an ongoing concern across very different eras.