The opening is just over a metre wide but only forty centimetres high, the kind of gap that would require a person to crawl, and beyond it a drystone passage curves gently away to the south or south-southeast. This is a souterrain, an underground stone-built passage or chamber constructed during the early medieval period, most commonly between the seventh and twelfth centuries. Their exact purposes are still debated, but storage and refuge are the most widely accepted explanations.

The souterrain sits within a rath, the Irish term for a circular earthen enclosure that served as a farmstead in early medieval Ireland. Raths are among the most common archaeological features in the Irish landscape, and it was not unusual for their occupants to construct souterrains beneath or within them, accessible from the interior of the enclosure. At Doonmaynor, the depression marking the entrance measures roughly 8.7 metres east to west and 7.8 metres north to south, sunk about 45 centimetres below the surrounding ground level. Large stones protrude from the base of this hollow on its eastern side, hinting at the extent of the stonework beneath. The passage itself is of drystone construction, meaning it was built without mortar, the stones carefully fitted to hold their shape across many centuries.