The entrance gives onto a passage or chamber running roughly four metres into the ground, cut directly from the earth rather than lined with stone. Beyond that, the picture gets murkier: a creepway extends toward the northwest, and to the northeast there appears to be a chamber that was never completed, or at least never finished in any conventional sense. Local accounts speak of "a couple of rooms inside", which tallies loosely with what can be seen from the threshold, but no formal access has been gained.

This is a souterrain, a type of underground structure built throughout early medieval Ireland, typically associated with nearby settlement sites. The word comes from the French for "underground passage", and these features were constructed variously for storage, refuge, or as escape routes connected to ringforts and farmsteads. They range from simple single chambers to elaborate multi-roomed systems with low connecting creepways designed to slow down any unwanted intruder. The Maulavanig example sits within that tradition, though the apparent presence of an unfinished chamber adds a small puzzle. Whether construction was abandoned, or whether what survives simply reflects damage and collapse over centuries, is not recorded. A circular depression about four metres in diameter sits on the far side of a field fence to the northeast, and this is likely the surface signature of a chamber that has fallen in, a reminder that what is visible above ground is rarely the whole story.