The larger of the two measures twenty feet long and four feet wide, roofed with flat flags and walled in stone, and it opens into a second room through an aperture just fifteen inches wide and ten inches high. Nobody has examined that second chamber, at least not in any recorded instance, because nobody could fit through the gap to reach it.

This is a souterrain, an underground passage or chamber, usually associated with early medieval ringforts, where it would have served for cold storage, refuge, or concealment. Souterrains of this kind are found across Ireland, typically hand-built from local stone and deliberately awkward to enter, their tight crawl-passages making pursuit difficult. The Kilcullen example sits in the south-western quadrant of a ringfort, a circular enclosed settlement of the early medieval period defined by an earthen bank and ditch. The details we have come from P. J. Hartnett, who recorded the site in 1939, describing what he found at the entrance and what could be glimpsed or inferred beyond it. The second chamber, he noted, appeared to be of similar construction to the first, though he could not get close enough to confirm it.

What makes the site quietly remarkable now is that none of this is visible at ground level. There is no surface trace, no indication that anything lies beneath. The measurements Hartnett recorded, the flagged roof, the stone walls, the constricted inner passage, all of it persists in the written record while the ground above gives nothing away.