The opening to the souterrain is still visible, and what lies beneath is more architecturally deliberate than the word "cave" might suggest. When Ordnance Survey fieldworkers noted it in 1839, they described it simply as "the entrance to a cave (the mouth of which is open)", which is a reasonable first impression of something whose full complexity only becomes apparent once you are inside.

A souterrain is an underground stone-built structure, typically associated with early medieval ringforts, thought to have served as a place of refuge, storage, or concealment. This one was first properly investigated in 1850, when a man named Windele had the entrance cleared and went in himself. His findings, along with a plan later drawn by S.P. Ó Ríordáin, formed the basis of a description published by McCarthy in 1977. The structure consists of an entrance passage leading to at least two chambers connected by creepways, the narrow low tunnels, sometimes barely 0.3 metres wide, that force anyone moving between chambers to crawl. Chamber 1 is irregular in plan, roughly 2.28 metres long and just one metre wide, with corbelled side walls, a technique in which stones are layered so that each course projects slightly inward to form a rough vault overhead. Its original entrance passage at the north-west end is now blocked, as is a creepway in the north corner. A second creepway at the south-east end connects it to Chamber 2, which is slightly larger at 2.6 metres long and more regular in shape. In Chamber 2, a stone bearing linear scores hangs from the roof about 0.3 metres out from the north-north-east wall, a detail whose purpose remains unexplained but which has attracted enough attention to be specifically recorded.