It measures roughly 11.5 metres north to south and 7 metres east to west, and its interior appears to be packed with stone. That detail, the stone fill, is what gives it away as something older and stranger than it looks.

What lies here is almost certainly a burnt mound, a type of prehistoric cooking or industrial site found widely across Ireland and Britain. The typical interpretation is that stones were heated in a fire, then dropped into a water-filled trough to bring the water to a boil, and the cracked, heat-shattered stones were discarded in a heap nearby. Over centuries, those discarded stones accumulated into exactly the kind of low, rounded mound visible at Lissooleen today. Most burnt mounds date to the Bronze Age, roughly 2000 to 500 BC, though some have produced earlier or later dates. They tend to cluster near water sources, which would have been essential to the whole process. The site at Lissooleen was recorded during a survey of the Lee Valley area carried out in 1996 and 1997.