At Cuillaun in County Mayo, a souterrain, an artificially constructed underground tunnel typically associated with early Irish ringforts and used variously for storage, refuge, or ventilation, has collapsed over time into an open, roofless depression. It is the kind of feature that rewards a second look, because at first glance it appears to be nothing more than a gentle dip in the ground.

The souterrain sits within the northern half of a rath, the circular earthwork enclosure that would once have defined a farmstead or settlement during the early medieval period. The passage itself is traceable as a grassed-over linear hollow, between one and one and a half metres wide and roughly sixty centimetres deep. From the rath's northern scarp it runs three metres to the south-southeast before turning and continuing southward for a further eight metres, giving the whole passage an irregular, angled course. At the northern end, remnants of stone facing remain visible, the only above-ground indication that this was once a carefully built structure rather than a natural feature.