Three large quarry pits, arranged in a rough north-to-south row, have been dug through the western half of the interior, each one extending inward from the perimeter towards the centre of the enclosure. The pits are ringed by loose stone and overgrowth, and the largest of them, the northernmost, drops to nearly two metres in depth. Someone, at some point, decided that whatever lay beneath this ancient enclosure was worth excavating.

A rath is a type of ringfort, typically a circular earthen enclosure dating from the early medieval period, used as a farmstead and defined by a bank and ditch. At Tullanacorra, the defining feature is a scarp rather than a raised bank, roughly circular and about 28 metres across, sitting on a level terrace on the south-western side of a ridge with long views opening out to the south-west. The structure is low and much worn, though a slight internal lip survives at the north-east. More intriguing is a feature marked simply as "Cave" on the Ordnance Survey six-inch maps of both 1838 and 1931, located in the western half of the rath. This appears to refer to a souterrain, an underground stone-lined passage associated with early medieval settlement, often used for storage or as a place of refuge. No trace of it survives above ground today. Whether it was destroyed by the later quarrying, or whether the quarrying was actually an attempt to find it or extract stone from it, is not recorded. The perimeter of the rath is now densely hedged with hawthorn, blackthorn, and brambles, and part of the northern and eastern scarp has been absorbed into a field fence running alongside a farm road. The site does not sit in isolation; at least three other raths lie within 500 metres, suggesting this small terrace in County Mayo was once a place of some significance in the early medieval landscape.