Measuring roughly twenty metres by seventeen and rising less than a metre from the surrounding ground, it is easy to mistake for a natural feature of the landscape. It is, in fact, the accumulated debris of prehistoric cooking, a fulacht fia, and it has been accumulating since well before recorded history in Ireland.

A fulacht fia is essentially a burnt mound, the by-product of a remarkably practical ancient technology. The method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water rapidly to the boil. The stones, cracked and spent after repeated heating and cooling, were raked out and piled to one side. Over generations of use, those discarded fragments built up into the low, rounded or horseshoe-shaped mounds that survive across Ireland in their thousands, almost always positioned near a reliable water source. The example at Acres fits the pattern precisely: the stream immediately to its east would have provided a constant supply of water. The mound's roughly rectangular shape and its modest but solid height of 0.85 metres speak to sustained, repeated use over time. Most burnt mounds in Ireland date to the Bronze Age, broadly between 2000 and 500 BC, though the form persisted in some areas beyond that period.