But Lough Oony, which has shrunk somewhat from its earlier extent, holds two such islands, and the northern one is a crannog, an artificially constructed or heavily modified island of the kind that served as a fortified dwelling or place of refuge across Ireland from the early medieval period onwards. Beneath the turf, erosion has exposed the island's core: a matrix of small angular stones packed with charcoal, the compressed remains of centuries of habitation and burning. The kerb stones that once defined its edge are still partially traceable, even if the piles and palisades that would once have risen from the water have long since disappeared.

The lough itself carries a considerable weight of recorded history. Known in older sources as Lough Uaithne, it appears in the Annals of Ulster as early as AD 719, when two sons of the King of Oriel were murdered there, a detail that places the site within the violent politics of early Irish kingship. In 851, a man named Cairil of Lough Uaithne was killed at the oratory of St Tigernach's church in Clones, suggesting that the lough gave its identity to a local dynasty or territorial grouping. By 1025, the shore had become the scene of a raid from Fermanagh in which seventeen men were killed. In the Middle Ages, Lough Oony appears to have been a centre of the MacMahons, one of the dominant Gaelic families of Monaghan, which may explain why the crannog and its southern neighbour were considered worth preserving in the landscape through successive centuries. Ordnance Survey maps from both 1834 and 1907 record both islands clearly. A log causeway connecting the northern crannog to the shore roughly 30 metres to the north was constructed in 1983, making the island accessible, though deep water surrounds it, particularly to the west and north.