When the field was ploughed in the recent past, the disturbance brought up a telling combination: very dark earth and quantities of burnt stone, the signature left by a fulacht fia. These are among the most common prehistoric monuments in Ireland, understood to be Bronze Age cooking sites where stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil. The mound itself is the accumulated debris of that process, cracked and fire-shattered stone heaped up over many episodes of use.

The mound at Glasnamullen measures roughly fourteen metres north to south and twelve metres east to west, rising to a modest height of about 0.7 metres. On its western side, an elongated oval hollow, seven metres long, four metres wide, and roughly 0.4 metres deep, cuts into the mound; this hollow likely marks the position of the original trough, whether timber-lined or simply cut into the ground, where the heated stones were plunged into water. The site sits on the eastern bank of the stream, which would have provided a reliable water source for filling that trough. Two further fulachta fiadh have been recorded in the immediate vicinity, suggesting this stretch of ground saw repeated, perhaps seasonal, use over a long period.