Beneath that subtle difference lies a spread of burnt stone and charcoal roughly seven metres across, the remnants of a fulacht fia, one of Ireland's most common yet least understood prehistoric monuments. A fulacht fia is essentially an ancient cooking or heating site, typically consisting of a trough that would have been filled with water and brought to temperature by dropping fire-heated stones into it. The stones crack and shatter with repeated heating and cooling, and over time accumulate into the characteristic horseshoe-shaped mound of blackened, fractured rock that marks these sites across the Irish landscape.

This particular example came to light during ploughing, which stripped away enough of the surface to reveal the burnt material beneath. The land had once been boggy, and it is that former wetness that matters. Fulachtaí fia are almost always found near water or on low-lying ground where water could be easily accessed, and the boggy conditions that made this terrain marginal for later farming helped preserve the site over the centuries. When the area was visited in 1989, the monument was under pasture but still legible as a faint rise in the field. What makes the Graigueooly location particularly notable is its company: a second fulacht fia lies roughly a hundred metres to the south-east, and a ringfort, an enclosed farmstead of the early medieval period, sits about the same distance to the east. The clustering of these features suggests the area was used, revisited, and settled across a very long span of time, with the boggy ground both limiting and shaping what people did there.