An L-shaped depression traces an irregular path across the earthwork, with stones breaking randomly through the sod along its full length. The shape, the dimensions, and those protruding stones all point toward the same quiet conclusion: that this is most likely the collapsed remains of a souterrain passage.

A souterrain is an underground stone-lined tunnel or chamber, typically built during the early medieval period in Ireland, most often in association with a rath, which is a circular enclosure defined by an earthen bank and ditch that served as a farmstead or small settlement. The combination was common enough across the country, though what survives at Doogary is more elusive than most. The depression runs roughly nine metres along the north-north-east edge of the rath platform before making a sharp right-angled turn and continuing for a further eight metres into the rath interior, narrowing slightly as it does so. At its widest it measures around two and a half metres across and half a metre deep. That right-angled turn is a recognised feature of souterrain design, where passages were deliberately bent to slow or confuse anyone attempting to force entry, suggesting the original construction was both deliberate and considered. Without excavation, however, a degree of uncertainty remains, and the structure is recorded as a probable rather than confirmed souterrain.