It is a fulacht fia, a type of site found in considerable numbers across Ireland, and one that only reveals its purpose once you understand what you are looking at. The horseshoe shape is the giveaway: these monuments, dating predominantly from the Bronze Age, are the accumulated waste heaps of repeated fire-cracking. Stones were heated in a fire, dropped into a water-filled trough to bring the water rapidly to the boil, and then discarded when they shattered from the thermal shock. Over time, generations of cracked and blackened stone built up into the distinctive curved mound that survives today.

The example at Carrigonirtane is a substantial one. The mound measures fifteen metres in length and just over fifteen metres in width, rising to nearly two metres in height, with the characteristic opening, roughly five and a half metres wide, facing north-north-west toward the trough area where the cooking or processing activity would have taken place. A drain running east to west through the eastern side and into that trough area is still traceable, suggesting the site was deliberately engineered to manage water flow, drawing on the nearby stream. The precise function of fulachtaí fia has been the subject of considerable debate among archaeologists: boiling meat is the traditional interpretation, but proposals have included brewing, textile processing, and bathing. Whatever the activity, the labour involved in collecting, heating, and discarding thousands of stones over repeated use speaks to a site that saw sustained and organised effort.