It is a fulacht fia, one of the most common and least celebrated monument types in the Irish landscape, and this particular example retains enough of its original form to give a clear sense of what Bronze Age cooking, or possibly bathing or industrial activity, once looked like at ground level.

A fulacht fia typically consists of a horseshoe-shaped mound built up from the accumulated debris of repeated heating: stones cracked by thermal shock, and soil darkened by charcoal from countless fires. The mound at Rossnashunsoge measures roughly eight metres north to south and six metres east to west, rising to about half a metre in height, with a two-metre opening facing west. The hollow at its centre would originally have held a trough, most likely timber-lined or clay-lined, into which water was poured and then heated by dropping fire-cracked stones directly into it. Over time, the broken and spent stones were raked out and piled up around the trough, forming the distinctive mound shape that survives today. The wet hollow in which this example sits is characteristic: reliable access to water was essential to the process. Perhaps equally telling is the proximity of a second burnt mound lying roughly fifteen metres to the north-east, suggesting this was a place returned to repeatedly, rather than used just once.