It measures just over twelve metres long, nearly nine metres wide, and less than a metre high, with an opening of around five metres facing north-west. Unremarkable to the passing eye, it is in fact the remains of a fulacht fia, a type of prehistoric cooking or processing site found in great numbers across Ireland, typically dating to the Bronze Age. The characteristic form involves a trough, a hearth, and a mound of fire-cracked stone: water in the trough was heated by dropping in stones from the fire, the stones eventually cracking and becoming useless, and the discarded fragments accumulating over time into exactly the kind of low, scorched mound visible here.

The site at Knocknashannagh was recorded by Bowman in 1934, placing it within a tradition of early twentieth-century field archaeology in Cork that sought to catalogue what were then often dismissed as mundane landscape features. What makes this particular location quietly interesting is the proximity of a second fulacht fia, situated around two hundred metres to the north-east. The clustering of such sites is not unusual; fulachtaí fia tend to appear near reliable water sources, and the Athnaloingebaine River would have made this marshy ground an attractive location for repeated activity over generations. Whether the two sites were in use simultaneously or represent different periods of occupation is not recorded, but their nearness to one another in this damp river margin suggests the spot was well used.