A cashel is a stone-walled ringfort, the kind of enclosed farmstead that dots the Irish countryside from the early medieval period, and souterrains, which are dry-stone underground passages associated with such sites, are not unusual finds within them. What is unusual here is the persistent uncertainty. Local tradition holds that a souterrain runs beneath the cashel's interior, yet no one has been able to confirm it. The only physical clue is a shallow depression in the north-eastern quadrant of the enclosure, its southern edge defined by a low north-facing scarp, a slight dip in the ground that could indicate subsidence above a hidden passage, or could mean nothing at all.

Souterrains were typically used for storage and, in some interpretations, as places of refuge, their cool, dark interiors maintaining a relatively stable temperature year-round. At Magheraboy, the feature that might betray one's presence is subtle enough to read as simple uneven ground to an unpractised eye. There is no excavation record, no mapped alignment, no confirmed entrance. The cashel itself carries the reference number MA074-009, placing it within the broader archaeological record for Mayo, but the souterrain remains in a category of its own, a tradition waiting for evidence, a depression in the earth that raises more questions than it answers.