Beneath that turf, however, lies a spread of burnt and fire-cracked stone, the accumulated debris of prehistoric cooking activity stretching back thousands of years. What looks like a slight rise in a wet field is in fact a fulacht fia, the remains of an ancient outdoor cooking site of a kind found in considerable numbers across Ireland, particularly in low-lying, waterlogged ground close to running water.

Fulachtaí fia, the plural form, are among the most commonly recorded prehistoric monuments in Ireland. The typical method of use involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil, cooking meat or, as some researchers have proposed, serving other purposes such as bathing or textile processing. The stones, repeatedly heated and quenched, shatter over time, and it is this accumulation of cracked, burnt material that forms the characteristic horseshoe-shaped mound visible at many sites. The Ballygirriha example follows this pattern, its spread of burnt stone preserved in the damp ground beside the stream that would once have supplied the water essential to the whole process. What makes the location particularly interesting is that it does not stand alone: a second fulacht fia lies approximately forty metres to the south, suggesting this stretch of streamside ground was a place of repeated or sustained use in prehistory, perhaps across generations.