The smallest of the group gives the site its name: Rathbeg, meaning simply "little rath" in Irish, sits on a gentle rise with an inlet of the bay visible around 150 metres to the east. A rath is a roughly circular earthwork enclosure, typically dating from the early medieval period and most often associated with a farmstead and its surrounding bank and ditch. This one is modest in scale, measuring about 21 metres across, but its position within a wider pattern of settlement makes it quietly interesting.

The enclosure was already identified by name on Ordnance Survey maps of 1838 and again in 1922, which at least confirms its visibility and local recognition across nearly two centuries of mapping. The earthwork itself is defined partly by a slumped bank on its north-eastern and eastern sides, and elsewhere by a scarp, a near-vertical cut in the ground, that reaches about 1.2 metres in height at the south and is faced with stone along its southern and south-western stretch. The northern and western sections are much more worn and difficult to read. A gap of around three metres exists at the south, though whether this marks an original entrance or is simply later damage remains unclear. A modern field fence cuts straight across the interior, dividing it slightly west of centre, and a semi-circular terrace appended to the south-south-west sits roughly a metre lower than the rath floor, its function uncertain but its deliberate shaping apparent. The broader landscape context is what sets this site apart from an isolated curiosity: a second rath lies 130 metres to the north-north-east, another 100 metres to the east-north-east, and two more are visible approximately 300 metres to the south-east, suggesting this corner of north Mayo once supported a notably dense pattern of early settlement.