The ringfort at Ballyglass Middle sits just below the summit of a ridge, an oval raised platform roughly 43 metres north to south and 51 metres east to west, and its southern bank now doubles as the county boundary itself. A rath, to give it its Irish term, is an enclosed farmstead of the early medieval period, typically defined by one or more earthen banks and ditches; this one has been so thoroughly absorbed into the landscape of field divisions and county borders that disentangling the ancient from the modern requires some attention.

The southern arc of the enclosure survives in the most legible form, a bank some 2.8 metres wide with stone facing on its exterior and an internal height of 1.3 metres, rising to 1.8 metres on the outside. Its curvature is original, preserving the sweep of the rath, but the almost vertical profile of its sides suggests deliberate modification, most likely when it was pressed into service as a county boundary marker. Along the western to northern arc, the rath perimeter merges with a straight field fence running northeast to southwest, also stone-faced, which introduces a quite different geometry to the monument. The northern to southeastern section has fared least well; the bank there has been largely levelled, though a subtle curving undulation in the ground still traces its line. A shallow ditch of about 2 metres width runs outside the southern bank, and while a ditch of this kind could indicate an original fosse, the defensive or demarcating trench that commonly accompanied a rath, it appears more likely here to be a later field ditch, given how naturally it connects with those bordering the adjacent field fences.

The ridge position is not incidental. The southern slope falls away in long, gentle undulations, opening onto wide views across the plains of Roscommon. That outlook would have made the site strategically sensible in any era, whether for an early medieval farming household or for those later generations who found the old earthwork a perfectly serviceable place to draw a county line.