A crannog, the Irish term for a man-made or partially man-made island dwelling used from prehistory through to the early modern period, it sits on a marshy promontory with deep water to its north, west, and south-west, and a strip of wet, boggy ground ten to fifteen metres wide separating it from dry land to the east. The effect, whether by accident or design, is one of near-total concealment, the kind of site that rewards a map and some patience rather than a casual glance from the shore.

When archaeologist Aidan O'Sullivan examined the site and published his findings in 2004, he found a circular mound roughly twenty-three metres in diameter and less than a metre high, built from a mixture of stone, earth, sand, and charcoal. The interior of the mound was composed of small stones, while the outer edges gave way to clay and soft, sandy silt. Around the south and south-west perimeter, vertical wooden posts of birch and alder, between five and ten centimetres in diameter and spaced at eighty-centimetre intervals, had been driven into the ground in two concentric rings set about two to two-and-a-half metres apart. There was no evidence of horizontal planking between them, suggesting the mound was likely placed directly onto the soft silt of the lakebed and then ringed with these upright posts, perhaps to consolidate or define its edges. Johnstown Lough itself is part of the Dysart Lake complex, a chain of small lakes along the River Deel in County Westmeath, a low-lying, water-rich landscape that would have made crannog construction both practical and strategically sensible.