This is a rath, a type of enclosed farmstead built during the early medieval period, typically defined by one or more earthen banks and an outer ditch. The one at Rathmorrissy measures approximately 52 metres across its northeast to southwest axis, which would once have made it a reasonably substantial enclosure, though what survives today gives little sense of that original scale.

When surveyors visited in February 1984, the bank and its external fosse, the ditch dug around the outside of the enclosure, were already much degraded and heavily overgrown. The bank was best preserved along its southwestern and western arc; from the east around to the south, a large portion had been dug away at some earlier point. At the east side, a causewayed entrance gap roughly five metres wide remained discernible, the original break in the bank through which inhabitants would have passed. Collapsed field walls were visible just outside the fosse on both the eastern and northern sides, suggesting later agricultural activity had overlaid the older monument. Associated with the site is a possible souterrain, an underground stone-lined passage or chamber of the kind often found beneath early medieval raths, used variously for storage or refuge. On a return visit in April 1991, the condition had worsened: the bank had been partially bulldozed between the south and southwest, with the displaced material pushed directly into the fosse, effectively destroying a section that had survived intact just seven years earlier.