It does not appear on the 1838 Ordnance Survey six-inch map at all, only showing up as a small subcircular island on the 1920 edition. What it actually is, is a crannog, an artificial or artificially enlarged island of the kind built on Irish lakes from the early medieval period onwards as defensible homesteads. This one has survived in unusual structural detail, its oak timbers still visible, its layout still legible, in a landscape of low hills and boggy ground that has changed little around it.

The cairn itself measures roughly 26.5 metres north to south and 25 metres east to west, and around its edges the remains of a palisade of radially-split oak planks are set into the lower stonework. Beyond those, a ring of retaining piles sits just outside the cairn's outer limit. Large oak beams protrude from the structure at the south-east and north sides, some containing mortice joints with timber pegs still in place, the joinery of early medieval carpentry preserved under water and stone for over fourteen centuries. The objects recovered from the site give a sense of the people who lived here: an iron three-pronged fish spear, a spade blade, an axehead, a lead weight, a pewter flagon fragment, and a whetstone. Four dugout canoes were also recorded on the lake bottom nearby, the likely craft that connected this island to the shore. In 1989, dendrochronological analysis, which uses the annual growth rings in timber to establish precise felling dates, was carried out on two of the structural oak beams. One beam, with a ring sequence spanning 208 years, matched against established Irish tree-ring chronologies and returned a felling date of AD 609, placing the crannog's construction firmly in the early Christian period. The artefacts recovered are now held in the National Museum of Ireland.