This is a fulacht fia, a type of site found in the hundreds across the Irish countryside, typically recognised by a horseshoe-shaped mound of burnt and cracked stone beside a water source. The principle was straightforward: stones were heated in a fire, dropped into a water-filled trough, and the resulting heat used to cook meat or, as some researchers have argued, for bathing or brewing. The Earlsrath example adds a particular wrinkle to this picture, because what was excavated here was not part of the main mound but an isolated earth-cut trough sitting some twenty metres to the north-east of an already-known fulacht fia.

The trough came to light in 2007 during excavations carried out ahead of roadworks on the N9/N10 Kilcullen to Waterford road improvement scheme. Its dimensions were modest but precise: roughly 2.2 metres east to west, 1.7 metres north to south, and 0.61 metres deep, oval in plan with sloping sides and a flattish base. What filled it was a tightly compacted mix of charcoal and heat-shattered sandstone, the same kind of burnt material that characterises fulachta fia across Ireland, suggesting this separate trough was part of the same tradition of repeated, intensive heating and water-boiling. Two further fulachta fia were also excavated in the same area, making this a cluster of related prehistoric activity concentrated along what was evidently a well-used stretch of streamside ground. The excavations were published by Wren in 2010.