The small circular mound sitting in unusually deep water, between 3 and 6 metres, is a crannog, an artificial or artificially enlarged island of the kind built across Ireland and Scotland throughout the early medieval period, typically as a defended dwelling place. But the cairn itself, roughly 18 by 20 metres and just 4.5 metres high, appears to be only what remained after a much larger timber superstructure collapsed or was dismantled. Oak beams, mortised and carefully hewn, are still visible within the stones on the north and west faces of the mound, and on the lakebed nearby, between 10 and 12 metres to the north-northeast, a cluster of 5 to 10 heavy oak timbers, each up to 6 metres long and up to 40 centimetres wide, lies in the mud at depths of 5 to 6 metres. These are not driftwood.

When archaeologists surveyed the site in 1987 as part of the Crannog Archaeological Project, they found a very large number of massive worked planks buried in the silt to the south and west of the island, some exceeding ten metres in length, with evidence of sophisticated joinery. Their conclusion was that the timber structure originally enclosing or surmounting the island was many times the size of the stone cairn now visible above the waterline. Dendrochronology, the science of dating by tree-ring analysis, has since placed the construction of the site at AD 855, give or take nine years, squarely within the period of Viking activity in the Irish midlands. Lough Lene itself sits in County Westmeath, a region with strong early medieval associations. A bronze bell recovered from the crannog in the nineteenth century and now held in the National Museum of Ireland, catalogued as NMI 1881:535, adds a further layer of intrigue: such bells were closely associated with early Christian monastic life, raising questions about who exactly was living here, and what purpose the island served beyond simple refuge.