The entrance is small enough to require crouching, framed by a lintel stone in the manner typical of early medieval underground passageways, and it opens onto a passage that runs to the north-west before meeting a creephole, a deliberately narrowed section designed so that only one person at a time could pass through, often interpreted as a defensive feature. Beyond that constriction, the structure has collapsed entirely, and the passage is now inaccessible.

Souterrains of this kind are found across Ireland and are generally associated with the early medieval period, roughly the sixth to the twelfth centuries. They were constructed from dry-stone walling and large roofing slabs, typically built into a bank or slope and then covered over. Their purpose has been debated at length; they may have served for food storage, as places of refuge, or simply as secure underground chambers attached to a nearby settlement. The deliberately tight creephole at the north-west end of the Moanflugh example fits a pattern seen elsewhere, where the layout of the passage was shaped to slow or hinder anyone entering uninvited. The surrounding pasture at Moanflugh gives little away above ground, the way these sites so often do, the landscape offering no obvious sign of what lies just beneath the surface of the slope.