It is a rath, the Irish term for a roughly circular earthen ringfort, and though thousands of these enclosures survive across Ireland, most dating from the early medieval period between roughly the fifth and twelfth centuries, this one at Bunnasillagh has worn down to something close to a suggestion.

The rath measures around eighteen metres in diameter, which places it at the smaller end of the scale for this type of monument. What remains is a low bank tracing an arc from the south-east through the south and around to the west-north-west, with a natural scarp of the ground completing the circuit elsewhere. On the western side, a slight external berm, a narrow flat ledge between the main bank and any outer ditch or slope, is still just about legible in the landscape. Raths of this kind were typically used as enclosed farmsteads, protecting a family's dwelling and perhaps some livestock within a defined boundary. Over the centuries, ploughing, grazing, and the slow creep of drainage work have reduced many such sites to exactly this kind of low profile, present enough to register on a survey but no longer immediately readable as a structure.

What makes this particular site quietly worth noting is its position relative to Lough Hacket. Sitting on a rise adjacent to the lake shore, the enclosure would have commanded a reasonable view over the water, and proximity to a lough was a practical consideration for early medieval settlement, offering water, fish, and sometimes a degree of natural defence. The surrounding undulating grassland gives little away now, and the monument demands patience from anyone hoping to trace its outline on the ground.