An L-shaped depression, roughly a metre deep and just over a metre and a half wide, sinks into the western half of an earthen enclosure. It is, most likely, all that remains of a souterrain, an underground stone-lined passage built during the early medieval period, typically as a place of refuge, storage, or ventilation for a settlement above ground. Here, the roof has long since given way, and no stonework breaks the surface.

The depression follows two axes: a longer run of around ten metres oriented north to south, with a shorter arm of about six metres extending east to west from its south-eastern end, giving the whole feature its L-shaped form. It sits within a rath, the circular earthwork enclosure that was the standard farmstead of early medieval Ireland, identifiable nearby as a separate recorded monument. The relationship between the two is typical; souterrains are commonly found within or immediately beside raths, suggesting they served the household that lived within the enclosure's banks and ditches. At Moanmore, though, the structure underground has collapsed entirely, leaving only the shape of what was once there.