The basic principle of a fulacht fia involves heating stones in a fire until they are intensely hot, then dropping them into a water-filled trough to bring the water rapidly to the boil. The result is a shallow mound of discarded, broken stone and charcoal that builds up over repeated use, sometimes over generations. What makes this particular example worth attention is not its surface appearance but the detail preserved underneath it, revealed only when Bord Gáis Éireann's Pipeline to the West cut through the site in 2002.

Excavators Tony Bartlett and Kate Taylor recorded three distinct subsoil features once the mound material was stripped back. The trough itself was sub-oval, roughly 1.86 metres by 1.8 metres and 0.8 metres deep, with no surviving lining; when the excavation reached its base, water seeped in to a depth of about 0.1 metres, reflecting the level of a trapped water table that would have made the trough naturally self-filling in antiquity. Close by, only 0.6 metres to the south-east, was a possible roasting pit measuring 2.1 metres by 1.7 metres, its base lined with thin sandstone slabs arranged into a neat platform roughly 0.94 metres by 0.6 metres, with scorched subsoil beneath suggesting sustained, direct heat. The north-western edge of this pit had been cut into a natural outcrop of layered sandstone bedrock, with the uppermost layers pushed back to form what the excavators interpreted as a work platform where someone could stand or sit. A stone-lined hearth, measuring 1.6 metres by 1.2 metres and only 0.1 metres deep, sat just 0.25 metres from the roasting pit, and although its base showed clear evidence of in situ burning, it had been carefully cleaned out after its final use, leaving no ash or fuel remains from the last firing.

The site is not publicly accessible as a visitor attraction, and much of it remains unexcavated in the adjacent field to the north-east, where a recorded baulk section showed the mound continuing beyond the pipeline corridor. The excavation report, compiled by Denis Power and published on excavations.ie, remains the primary means of engaging with Ballynacragga in any depth. For those travelling through the area with an interest in what lies just beneath ordinary agricultural land, this site is a reminder that the quiet fields of County Limerick conceal a remarkably dense archaeological record, most of it still in the ground.