It is roughly circular, around 35 metres across and 4 metres high, and at first glance it might pass for a natural feature of the lake. It is, in fact, a crannog, an artificial or artificially enlarged island of the kind built and inhabited throughout Ireland from prehistory into the early medieval period, and the story compressed into its timber and stone substructure spans nearly three thousand years of continuous and intermittent use. What makes Coolure Demesne unusual even among crannogs is not just its age but the sheer range of people who seem to have wanted it, from Late Bronze Age communities to Viking traders, and the accumulation of lives it quietly holds.

Excavations carried out in 2004 by a joint team from UCD, the National Museum of Ireland, and the Underwater Archaeology Unit established that the island was first built and occupied around 850 BC, in the Late Bronze Age, when a roundwood-post palisade was raised around an island of clays and peats. Weapons and ornaments recovered from the nearby Kiltoom shoreline suggest the site may have served a ritual as well as a domestic function, perhaps as a place where offerings were made to the water. The island was then reactivated around AD 402, a date established through dendrochronology, the analysis of tree rings in the massive oak planks used to enclose its southern and western edges. This phase of use coincided with significant political upheaval in the Irish midlands during the 4th and 5th centuries. By around AD 650 the early medieval crannog had taken shape, with palisades driven into a mound of peat and stone, and it may by that point have been a royal or lordly residence of the Uí Fiachrach Cúile Fobhair, a dynasty whose seat historical sources place on the northern shores of Lough Derravaragh. The midden of cattle, pig, horse and deer bone found around the southern margins of the island points to high-status feasting; palaeoenvironmental analysis found evidence of oats and barley, probably consumed as porridge or gruel; and insect remains indicated, less glamorously, that the island's edge served as a midden and latrine. Iron arrowheads designed to pierce chain-mail, a bishop's seal matrix, and Anglo-Norman jewellery extend the occupation record into the 13th century. Many of the most striking artefacts, including Viking silver armlets, weighing scales, and ingots, almost certainly connected to trade with Viking Dublin, were removed by metal detector enthusiasts in the 1980s before reaching the National Museum. A small ruined structure in the northern quadrant of the island, roughly 3.7 metres by 2.5 metres, belongs to a much later chapter entirely: it was probably a summer house associated with Coolure House, a Georgian residence some 520 metres to the north-northeast, and the island was still being used as a fishing lodge and a local swimming spot until drainage works altered the shoreline in the 1960s.