In the south-western corner of a ringfort near Cloonboorhy, a stone-lined depression running north to south marks the collapsed roof of a souterrain, an artificially constructed underground chamber or tunnel typically associated with early medieval ringforts across Ireland. Souterrains were usually built from drystone walling and roofed with large capstones, then covered with earth; when those capstones eventually give way, the surface drops and the outline of the passage becomes readable from above, which is precisely what has happened here.

The ringfort itself, the circular earthwork enclosure within which this feature sits, belongs to a type of settlement that was common in Ireland roughly between the sixth and twelfth centuries. Souterrains attached to such sites are thought to have served as places of refuge, cool storage for dairy produce, or both. The Cloonboorhy example sits in the south-western quadrant of its parent ringfort, a placement that is not unusual, as souterrain entrances were often positioned close to the interior of the enclosing bank. The site was documented in a 1994 archaeological survey of the Ballinrobe district, an area that encompasses the landscapes around Lough Mask and Lough Carra, compiled by D. Lavelle for the Lough Mask and Lough Carra Tourist Development Association.