Souterrains are underground stone-built passages associated with early medieval ringforts, and they were used for storage, refuge, or concealment. What makes the one at Oranhill quietly remarkable is the complexity of its internal arrangement and its sheer scale: the overall structure runs to more than 30 metres in length, placing it among the larger examples of its kind.

The souterrain sits in the north-eastern quadrant of a rath, the circular earthen enclosure, known separately in the record, that would once have been the defended homestead of an early medieval farming family. The underground structure was built using the drystone method, meaning the walls were carefully laid without mortar, and the roof was formed from flat lintels spanning the chamber walls. Three separate access points exist, each found through gaps between those roof stones. The first chamber, oriented roughly north-northeast to south-southwest, is largely collapsed, with only about 2.7 metres of its original 6.7-metre length still intact. A creep that once connected it northward to the second chamber has been blocked. The second chamber, running west-northwest to east-southeast, is better preserved at around 8 metres long. From it, a drop-hole creep, essentially a deliberate step down through a very tight passage, leads into the third and final chamber. This chamber is the most damaged: only its south-western end survives intact, while a surface depression stretching roughly 10 metres marks where the rest of it has fallen in, with the tops of the original side walls still visible in the edges of that hollow. A small recess built into the wall of the third chamber, the function of which is not certain, adds one further detail to an already intricate design. The site was noted by Athy in 1914 and again by McCaffrey in 1952.