It measures somewhere between 65 and 70 metres in circumference and is built from boulders, with the remains of a timber palisade still partly visible along its northern to south-western edge. Seventy-three wooden posts, ranging in diameter from around ten to twelve centimetres, survive at varying heights, some barely level with the lake bed, others rising to a full metre on the southern side. This is a crannog, a type of artificial or partly artificial island used throughout Ireland and Scotland from the Bronze Age into the early medieval period, typically as a defended dwelling place, and this one has been quietly eroding into its lake for centuries.

The island appears on Ordnance Survey maps from 1842 and 1904 under the name Illaunyweahagane, a corruption of the Irish Oilean Uí Mhaothagain, meaning Meighan's island. The scholar O'Donoghue, writing in 1986, traced that name to a chieftain of the Cineal Laoghaire, a sept with deep roots in this part of mid-Cork. By 1909, a local writer named Lunham could still observe many of the wooden piles of the substructure and, more intriguingly, described a raised passage running above the level of the lake bed, connecting the island to a rath, a roughly circular earthwork enclosure, somewhere near the shore. When Tony Balfe carried out a brief inspection in January 1994, working under licence from the Office of Public Works, no trace of that passage could be found. The centre of the island was by then completely inaccessible due to overgrowth, and on the western side a spread of boulders had collapsed over the palisade, obscuring whatever structural detail might have remained beneath.