That kind of deliberate subterranean construction, built without mortar from carefully stacked dry stone, is known as a souterrain, a term borrowed from the French for underground passage. These structures appear widely across early medieval Ireland, typically associated with ringforts, and are generally understood to have served as storage spaces, refuges, or both, taking advantage of the stable cool temperatures underground.

The souterrain at Caherduff sits at the centre of a ringfort, the circular earthwork enclosure that would have defined a farmstead or settlement during the early medieval period, roughly between the fifth and twelfth centuries. Two chambers were built here, connected or arranged in the dry-stone manner typical of such construction. A 1994 archaeological survey of the Ballinrobe district, compiled by D. Lavelle and covering the areas around Lough Mask and Lough Carra, recorded the site, noting that both chambers had by that point largely collapsed and become infilled over time.

What survives at Caherduff is fragmentary at best, the kind of site that rewards those already curious about the texture of the early medieval landscape rather than those looking for visible drama. The ringfort itself remains the primary above-ground feature, and the souterrain beneath it is more a presence felt in the historical record than one easily traced on the ground today.