A rath is an earthen ringfort, the most common monument type in Ireland, typically built during the early medieval period as an enclosed farmstead for a single family and their livestock. This one at Ballyglass measures around 33 metres north to south and 34.5 metres east to west, defined by an earthen bank that still stands over two metres high on its southern exterior face. The views it commands are considerable, looking out over undulating ground to the north and east, and across a broad valley of reclaimed damp pasture to the south and west.

The engineering choices embedded in the structure are telling. On the south-western arc, where the ground naturally falls away, the bank was built up higher to compensate, keeping the enclosure level and presenting a consistent defensive or symbolic profile to the outside. Along the inner face of the bank, stones and boulders have been incorporated at various points, and looser field clearance stones have accumulated against the inner slope on the south-eastern side. Just outside the bank to the south-west, a shallow depression and a faint outer rise suggest that a fosse, a defensive ditch, once ran around this section, since infilled and largely levelled. The most probable location for the original entrance lies on the east-south-east arc, where the bank dips slightly, though the gap is not sharply defined. Several other breaks around the circuit are the more recent work of farm animals rather than any ancient gateway. A small crescent-shaped stony mound near the centre of the interior may be nothing more than a field clearance heap, accumulated over generations of working the ground. A few hawthorn trees have taken hold on the bank itself, and an old farm track running just to the north leads to a derelict farmstead, a reminder that the landscape around the rath has been worked and reworked long after the ringfort's original occupants were gone. Roughly 260 metres to the north-east, another rath survives, suggesting this part of Mayo once held a cluster of early medieval settlement.