This is a souterrain, an underground passage or chamber of drystone and earthen construction, typically associated with early Irish ringforts and used variously for storage, refuge, or purposes still debated by archaeologists. The one at Cnoc An Bhróigín Thiar is now inaccessible as a whole, but its partial survival tells enough of a story to be worth the attention.

The ringfort itself, known as Lisnahorna or Lios na hEornan, is a univallate rath, meaning it is enclosed by a single earthen bank and fosse (a ditch dug around the perimeter). Within its interior there is also a possible hut-site, though the souterrain is the more legible feature. The underground structure runs in a rough Z-plan, beginning at the south with what appears to be the original entrance, an earth-cut well opening into a lintelled passage just 0.6 metres wide and 0.6 metres high. Beyond this tight threshold lies a slightly wider space, its western wall curved and built in drystone, before collapse blocks further progress after about 1.5 metres. A short passage extends east-southeast, its walls formed mainly from large upright slabs with drystone filling the gaps, and beyond further collapse, a final passage runs west-northwest for at least 2.5 metres, measuring 0.65 metres high and 0.5 metres wide. The detail comes from J. Cuppage's 1986 archaeological survey of the Dingle Peninsula, the Corca Dhuibhne survey, which documented this and hundreds of other sites across one of the most densely layered landscapes in Ireland.

Two openings at ground level allow a partial view of the structure without entry. The dimensions recorded give a sense of what the original builders intended: this was not a space meant for standing upright or moving quickly, but for concealment, for cold storage, or perhaps for both. The surrounding bank and fosse of the rath remain visible on the slope, and the narrow valley below still drains quietly toward the Milltown river, much as it would have when someone last ducked through that lintelled gap.