It is, in fact, a burnt mound, a category of prehistoric monument found widely across Ireland and Britain, and one of the more enigmatic survivors of early human activity. Beneath its sod covering lies a mass of shattered sandstone packed into a charcoal-rich matrix, the accumulated debris of what archaeologists believe were episodes of intense, repeated heating and cooling of stone.

Burnt mounds, sometimes called fulachtaí fia in Irish, are generally understood to be the byproduct of an ancient cooking or heating method in which stones were fired in a hearth and then dropped into a water-filled trough, rapidly bringing the water to a boil. The stones crack and fragment with thermal shock, and once spent they are discarded, building up over time into the characteristic mound shape. The Bunnafedia example is oblong, measuring roughly 18 metres north to south and 12 metres east to west, and rising to about half a metre at its highest point. Its position is telling: it sits at the edge of a natural scarp bordering an area of wet, marshy ground, which would have provided a reliable water source. The mound slopes down towards that marshy ground on its western side, with a well-defined edge to the north and north-east, while its southern limits are harder to read in the present landscape. A low knoll to the south-west offers some shelter, though that knoll has since been partly quarried away. Notably, a second burnt mound lies approximately 140 metres to the north-east, raising the possibility that this part of Sligo saw sustained or repeated use over time.