At Lismacbryan, a semi-circular mound measuring roughly twelve metres north to south and nine metres east to west sits at the base of a west-facing ridge, beside a field drain. The mound itself would be easy to dismiss as a natural irregularity in the ground, but a cross-section exposed in both faces of that drain tells a different story: a layer of burnt stone and charcoal, over eleven metres long and up to eighty centimetres deep, accumulated deliberately against the slope of the ridge over what must have been repeated use.

Burnt mounds are among the more quietly puzzling features of the Irish prehistoric landscape. They are the by-product of a process in which stones were heated in fire and then plunged into water, either to cook food, process hides, or produce steam in a sweat-house type structure. The cracked, fire-shattered stones were discarded in a heap after use, which is why the mounds are typically crescent or horseshoe-shaped, built up around a central trough or pit. Most date to the Bronze Age, roughly 2000 to 500 BC, though precise dating for individual sites requires excavation. The location at Lismacbryan is typical of the type: low-lying, close to a water source, and beside wet pasture that would have provided reliable access to water. A second possible example of the same feature lies around a hundred metres to the south-east, suggesting this was not an isolated episode of activity but perhaps a locale returned to over time.