Beneath the surface lay spreads of greyish-black silty clay packed with charcoal and heat-shattered stone, the characteristic signature of a burnt mound. These ancient features, sometimes called fulachta fiadh, are among the most common prehistoric monuments in Ireland, though their precise function remains debated. The leading theory is that they served as cooking sites, where stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil, but other uses, including bathing or industrial processing, have been proposed. Whatever their purpose, they speak to sustained, organised activity at a fixed location, probably during the Bronze Age.

The largest feature identified at Illaunyregan, designated Burnt Mound 1, was sub-oval in plan, measuring roughly 23.5 metres northwest to southeast and 3.8 metres northeast to southwest. It extended beyond the northern limit of the works area and was preserved in situ rather than excavated. Three smaller spreads of the same material lay approximately two to five metres to the south and were fully excavated in September 2021 by F. Walsh. These had been disturbed by more recent activity but are thought to represent detached portions of the same original mound. One of these adjacent spreads yielded a stone axe fragment, a small but suggestive find that hints at a broader pattern of prehistoric use in the area. Adding further weight to that picture, a second burnt mound sits roughly 55 metres to the northwest, suggesting that this stretch of ground in Illaunyregan was a place people returned to, and worked at, over considerable time.