The souterrain, an underground stone-lined or earthen tunnel typically built during the early medieval period as a place of storage, refuge, or concealment, has collapsed so thoroughly that nothing of it is visible from the surface today. What was once an entrance has closed over, and the ground above gives no indication that anything lies beneath.

When P. J. Hartnett visited in 1939, there was still just enough left to describe. He recorded a round opening, roughly three feet across and six feet deep, with loose collapsed earth filling whatever lay below it. A short passage extended from this shaft toward the rampart of the surrounding ringfort, and he noted specifically that neither the passage nor the opening showed any masonry. That detail is worth pausing on: many souterrains were carefully lined with dry-stone walling, but this one appears to have been entirely earthen in construction, which may partly explain why it gave way so completely. The ringfort itself, a roughly circular enclosure defined by one or more earthen banks and ditches and a common feature of early medieval Irish settlement, provided the broader context for the souterrain; such tunnels were frequently built inside or adjacent to the ramparts of these enclosures.

By the time Hartnett's observations were published, the site was already in poor condition. Today, with no visible surface trace remaining, the souterrain at Rathcoola exists almost entirely as a record of what was briefly glimpsed before the earth reclaimed it.