Beneath the turf, however, lies a spread of burnt and shattered stone, the defining signature of a fulacht fia. These are among the most common prehistoric monuments in Ireland, typically Bronze Age in origin, and the current understanding is that they functioned as outdoor cooking sites. The basic principle involved heating stones in a fire, dropping them into a water-filled trough to bring it to a boil, and repeating the process until food was cooked. The stones, cracked by the thermal shock, were discarded into a mound around the trough, and it is precisely this accumulation of fire-reddened and fractured material that survives at Rossnanarney today. Some of the burnt stone has also been dumped along a field fence roughly five metres to the east, suggesting the site has been disturbed or partially cleared at some point.

What makes this particular location quietly compelling is its relationship to the surrounding landscape. A second fulacht fia lies approximately 170 metres to the south, on the opposite bank of the same stream, and a possible third example has been identified somewhere to the north, though its precise location has not been pinned down. The clustering of these sites along a watercourse is entirely characteristic. Fulachtaí fia were almost always positioned close to a reliable water source, both for practical reasons and, perhaps, because streams and wetlands carried their own significance in prehistoric thought. Whether the examples at Rossnanarney represent successive use of the same general locality over time, or activity by different groups drawn to the same useful geography, is not known.