But the stream has been doing slow, inadvertent archaeological work on its eastern bank, eroding the mound's edge and exposing what lies within: shattered sandstone fragments embedded in a black, charcoal-rich soil, and, roughly halfway down the exposed face, the visible edges of four wooden planks laid side by side. Those planks are the base of a trough, and the mound itself is a fulacht fia, a class of prehistoric cooking site found in considerable numbers across Ireland, typically consisting of a horseshoe-shaped mound of fire-cracked stone accumulated beside a water source, with a timber- or stone-lined trough dug nearby to hold water that was heated by dropping fire-warmed stones into it.

The Ballyglass example is semi-circular in plan, measuring twelve metres north to south and nine metres east to west, with its central hollow oriented towards the stream on its eastern side. The mound rises roughly half a metre above the surrounding ground on its western edge, and about a metre above the stream bed on the eroded east side, where the section face is most legible. The trough timbers sit at a depth of 0.9 metres below the top of the mound, protruding only a few millimetres from the cut face, just enough to be identified as four contiguous horizontal planks spanning a width of 1.5 metres. The soil immediately above them is peaty, with relatively little burnt stone, while the material to the north and south is densely packed with it, a pattern consistent with stone being heated, used, and discarded repeatedly over time around a central working area. Scattered through the mound material, a fragment of hazelnut shell was also noted, a small organic remnant of whoever was using this place, at whatever remove of prehistory.

The hillock to the south-west and the gently rising ground to the west and north-west would have made this a reasonably sheltered spot, close to a reliable water source. The mound is largely sod-covered and sits in ordinary farmland, and some burnt stone is also visible in section on the opposite, eastern bank of the stream, suggesting the site's footprint may be wider than the mound alone implies.