A crannog is an artificial or partly artificial island, typically built during the early medieval period as a defensive or high-status dwelling place, and this one at Rahasane presents as a circular, tree-covered cairn of stones roughly thirty metres across and three metres high, bounded at its edge by a low drystone wall of large blocks. The island sits about fifteen metres south of a channel of the Dunkellin, rising out of ground that floods and drains with the seasons, as turloughs do.

The site came to the attention of local antiquarians in the early twentieth century, recorded by Redington in 1916. What gives it an additional, more recent dimension is a small rectangular enclosure near its summit, only about three metres long and a metre and a half wide, built of drystone walling just a single course high. This modest structure is almost certainly not ancient. McCaffrey, writing in 1952, suggested it was most likely built by duck-hunters who used the island as a hide, crouching behind the low wall to wait out the wildfowl attracted to the surrounding wetland. It is a quietly telling detail: a prehistoric or early medieval island refuge, repurposed without ceremony by men with shotguns, the older archaeology simply incorporated into the newer concealment.