A burnt mound, known in Irish archaeology as a fulacht fiadh, is typically a low mound of heat-shattered stone and dark, charcoal-stained earth, the accumulated debris of a process involving repeatedly heating stones in a fire and dropping them into a water-filled trough, most likely for cooking, bathing, or craft processes such as textile working. They are among the most common prehistoric monuments in Ireland, yet each new find adds another point to a map of ancient activity that is still far from complete.

The main burnt mound uncovered at Illaunyregan is subrectangular in plan, measuring roughly 4.5 metres east to west and 3.7 metres north to south, and it extends beyond the north-eastern limit of the works area, meaning a portion of it remains unexcavated and was preserved in situ. About 7 metres to the west, a smaller and more irregular spread, roughly 2.5 metres in diameter, was fully excavated in September 2021 by F. Walsh, and is thought to represent displaced or secondary material from the main mound. What makes the location particularly notable is that another burnt mound already on record lies approximately 55 metres to the south-east, suggesting this stretch of ground saw repeated or prolonged use during prehistory. The cluster of activity around a single area is not unusual for burnt mounds, which often appear in low-lying, waterlogged ground close to streams or marshy areas, precisely the kind of landscape that flood relief schemes are designed to manage.