Beneath the surface, or what remains of it, lies a souterrain, an underground stone-built passage or chamber constructed in early medieval Ireland, typically as a place of refuge, storage, or concealment. The chamber here has largely collapsed, but the fallen lintels still give a sense of the careful corbelled or lintel construction that once made these underground spaces functional and relatively secure.

The souterrain sits within the eastern half of a ringfort, the circular earthwork enclosure that formed the basic unit of rural settlement in early medieval Ireland, usually surrounding a single farmstead. That pairing is entirely typical. Souterrains were frequently built inside ringforts, accessible from within the protected interior, and the combination of above-ground enclosure and below-ground chamber reflects a practical approach to both defence and storage in an era before more permanent architecture. The site at Knocknamucklagh was recorded in a 1994 archaeological survey of the Ballinrobe district compiled by D. Lavelle, which catalogued monuments across the Lough Mask and Lough Carra area, a region whose landscape holds a considerable density of such early medieval remains.