The site came to light during excavations carried out by Brendán Ó Riordáin as part of the Arklow bypass road scheme, recorded under excavation licence 97E0083. What he found in that irregularly shaped hollow, measuring roughly 0.9 metres by 0.5 metres and 0.6 metres deep, was a spread of burnt mound material along with tiny fragments of burnt bone.

Burnt mounds are among the most common prehistoric monument types in Ireland, yet they remain quietly unfamiliar to most people. They typically consist of fire-cracked and heat-shattered stones, discarded after repeated use in heating water, and they are usually associated with cooking, though some archaeologists connect them with bathing or industrial processes. The presence of burnt bone at Johnstown adds a further layer. It does not necessarily point to a burial; small bone fragments of this kind can result from the disposal of food waste or from activities carried out around an open hearth over a long period. The site was recorded as Site O within the broader excavation, suggesting it formed one component of a more complex archaeological landscape uncovered during the bypass works.