It measures roughly 11.7 metres on its north-north-west to south-south-east axis and 13.1 metres across, rising to a height of about 1.15 metres. That is not a natural rise in the ground. It is the accumulated debris of repeated prehistoric cooking, the kind of site archaeologists call a fulacht fia.

A fulacht fia is a burnt mound, typically Bronze Age in origin, formed around a trough into which water was channelled and heated stones were dropped to bring the liquid to a boil. The stones, cracked and useless after a single heating, were raked out and discarded, and over many uses they built up into the horseshoe-shaped or oval mounds that survive across Ireland in their thousands. The process was efficient enough to cook large quantities of meat, and some researchers have proposed additional uses ranging from textile processing to bathing. At the Tullylease example, a spring lies to the north-north-west of the mound, which fits the pattern well. Water sources are almost always found close to these sites, since the whole operation depended on a reliable supply. The spring here was almost certainly what made the location worth returning to, season after season, by whoever was using this slope during the Bronze Age.