Excavations carried out ahead of the build revealed a fulacht fia, one of the most common prehistoric monument types in Ireland, though still imperfectly understood. A fulacht fia typically consists of a mound of fire-cracked stones accumulated beside a water trough, the working debris of a process that involved heating stones in a fire and dropping them into water to boil it. What they were used for, whether cooking, hide-working, bathing, or something else entirely, remains a matter of genuine archaeological debate.

The mound at Stag Park was relatively modest in height, rising only about 0.2 metres, but spread across a footprint of roughly 19 by 15 metres, packed with heat-shattered stone and charcoal-darkened soil. Beneath its northern end, excavators uncovered a rectangular timber-lined trough, its original lining long gone but its former presence betrayed by eight stake-holes set into the corners. To the south of the trough lay a larger pit, measuring over four metres in length and surrounded by twenty-five stake-holes and three post-holes, the remnants of some kind of timber structure whose precise function is unclear. Notably, this pit contained no burnt stones at all, which suggests it had been deliberately filled in before the mound of fire-cracked material was allowed to accumulate around it. A second, smaller pit cut into the north-western end of the first, further complicating the sequence of activity on the site. Radiocarbon dating placed the site firmly in the early Bronze Age, with a calibrated date range of approximately 2122 to 1828 BC. A second fulacht fia lay just five metres to the north-west and was excavated at the same time, raising the possibility that this was an area of repeated or sustained prehistoric activity rather than a single isolated episode.