What makes this particular site worth pausing over is not just its condition but a small oval depression cut into its south-western interior, roughly eight and a half metres long and just under four metres across at its widest point. That hollow, sunk to an average depth of about 0.9 metres and oriented east to west, is identified as a possible souterrain, an underground passage or chamber typically stone-lined and covered, used in early medieval Ireland for storage, refuge, or both. The combination of a well-defined enclosure and a probable subterranean feature in a single compact site gives Killagh Beg a quiet layered quality.

The earthwork itself is a rath, the most common type of Irish ringfort, built as a farmstead enclosure during the early medieval period, roughly between the fifth and twelfth centuries. This one is subcircular, measuring approximately 32.6 metres east to west and 31.6 metres north to south, and is defined by two concentric banks with a fosse, or ditch, running between them. A double-banked arrangement of this kind was generally associated with a household of some local standing; single-bank raths were far more typical. The entrance faces east-south-east, the direction most commonly favoured in Irish ringfort construction, likely for reasons of morning light and prevailing wind. Notably, the site sits only 150 metres north-west of another ringfort, suggesting that this part of the hill was settled with some density, and that the two enclosures may have been occupied at the same period or in close succession.