It is barely thirty centimetres at its highest point, roughly eight metres long and six metres wide, and its edges dissolve so gradually into the surrounding field that there is no clear moment where it begins or ends. Only when you probe beneath the surface does the picture change: small fragments of burnt stone embedded in black soil, the characteristic signature of a fulacht fia.

A fulacht fia is a type of prehistoric cooking or industrial site found in enormous numbers across Ireland, typically dating from the Bronze Age. The usual interpretation is that stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil, used for cooking meat, processing hides, or possibly bathing. The shattered, fire-cracked stones were discarded in a heap around the trough, and over centuries these heaps accumulated into the low, kidney-shaped or oval mounds that survive today, often in low-lying or waterlogged ground. The Ballyglass example fits this pattern closely. It sits on a narrow strip of marshy land, bordered to the south by a field drain running roughly east to west, with a farm road just beyond it. A low ridge rises to the north-east and east, providing a degree of shelter. The wet, boggy setting is entirely typical; water was a practical necessity for the whole process, and such spots were often chosen precisely because they retained it.