It measures roughly five metres north to south and four metres east to west, and it is made almost entirely of burnt material, the accumulated debris of repeated ancient fire and water. That modest heap is a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland, typically identified by exactly this kind of scorched, fire-cracked stone mounded beside a water source.

The term fulacht fia, sometimes rendered as fulacht fiadh, refers to what archaeologists understand as an outdoor boiling site, most commonly dated to the Bronze Age, though examples span a wide range of periods. The usual interpretation is that stones were heated in a fire, then dropped into a water-filled trough to bring the water to a boil, with the cracked and spent stones discarded to the side over time, gradually forming the characteristic horseshoe-shaped or irregular mound that survives today. The site at Glantane fits this pattern closely: boggy, low-lying ground beside running water, with the burnt mound as the only visible surface trace of what were probably repeated episodes of use over many years. Ireland has thousands of recorded fulachta fiadh, making them among the most common prehistoric monument types in the country, yet individually they attract little attention, partly because they rarely look like much from the surface.