A crannog, in essence, is an artificial or partly artificial island, built up from timber, earth, and debris, used as a defended dwelling place from the early medieval period and sometimes earlier. The Clonickilvant example is unusual not because of what survives, but because of what was done to it before anyone thought to look carefully. The interior was thoroughly ransacked, by local people and antiquarians alike, in pursuit of what was hopefully referred to as "Danes' treasure", the persistent folk belief that Viking-era riches lay buried beneath such mounds. The centre was effectively ploughed out, probably in the early twentieth century, and whatever stratigraphic sequence once existed there is long gone.

The site has a quietly melancholy cartographic history. The 1837 Ordnance Survey six-inch map shows it as a distinct triangular island sitting within the lough. By the time the revised twenty-five-inch edition was produced in 1913, the water level had dropped by roughly a metre, and what had been an island had become a rounded projection of the shoreline, landlocked on its western, northern, and north-eastern sides. When the mound was examined in 1981, it measured approximately 44 metres north-east to south-west and 36 metres north to south, rising about 1.4 metres above the then water level. Up to seventy stakes and piles were visible on the eastern side, arranged in two roughly concentric rings, with split planks and at least one piece of timber retaining fine woodworking detail, including a small preserved peg. Animal bone was scattered throughout, but no objects of note remained in situ. Earlier disturbance had, however, yielded a portion of a large dugout canoe hollowed from a single oak, a bronze pin, a crucible, and a stone axe, with numerous other finds said to have been deposited in the Royal Irish Academy museum. The site sits on a barony boundary, within a wider drained wetland system that once included Black Lough and the raised bogs surrounding it, suggesting this was once a more extensive and ecologically complex landscape than the present farmland implies.