What lies beneath it is rather more interesting. When workmen began pulling up an old stone fence running the length of the field at Ballydunlea some time in the summer of 1854, they broke into an underground passage that had, by all appearances, been sealed and undisturbed since its original occupants had last used it.

The discovery was recorded by a Mr Hitchcock, who wrote about it in 1854 to 1855 in enough detail to make the scene vivid. The property belonged to the Messrs. Hilliard, and it was Hitchcock himself who retrieved a few fragments of bone from the inner chamber. What he found was a souterrain, a type of stone-lined underground structure built during the early medieval period in Ireland, typically associated with ringforts and used for storage or refuge. The entrance opened from the west into a long passage that sloped downward to meet an inner chamber, with the whole axis running roughly east to west, the same line as the old fence that had been laid directly over it. Hitchcock examined the lintels, the flat stones forming the roof of the passage, carefully enough to note that none carried any inscription. Above ground, the situation was more ambiguous. The overgrown mound was visible, and Hitchcock thought it might once have been part of a fort's embankment, but even then the earthwork had lost enough of its shape that nothing could be confirmed. No definite trace of a fort survived at the surface. In a neighbouring field, closer to the river, he also noted what he described as an interesting fort, suggesting the broader landscape around Ballydunlea once held considerably more activity than the surviving remains now suggest.