Beneath the sod, however, the soil is black and rich with charcoal, and wherever erosion has broken through the surface, heat-shattered stones are exposed. This is a burnt mound, a type of prehistoric site found in considerable numbers across Ireland and Britain, and one whose function still prompts debate among archaeologists.

Burnt mounds are generally associated with the Bronze Age, and the leading theory holds that they were cooking sites, where stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil. Repeated heating and sudden cooling causes the stones to fracture and become unusable, so they were discarded into a pile nearby, accumulating over time into the characteristic mound of cracked, fire-blackened rock. The Shanvally example is oblong in plan, running roughly ten metres on its longer axis and about four metres across, and rises to somewhere between half a metre and just under a metre in height. It sits in wet pasture, which is typical; burnt mound sites are almost always found close to water or in low-lying, boggy ground, both because water was central to their use and because such marginal land was less likely to be ploughed away over the centuries. A silted-up field drain, about 1.2 metres wide, has cut across the eastern edge of the mound at some point, and a parallel field bank runs alongside it. The shattered stones visible in that bank suggest material from the mound was simply incorporated when the drain was dug, a small indignity that has, at least, preserved a cross-section of the site's interior for anyone who looks closely enough.