It measures roughly 39 metres east to west and 37.5 metres north to south, and what defines it is not a dramatic wall but a combination of a scarp, faint traces of a bank, and an outer fosse. A fosse is simply a ditch, dug to reinforce the enclosure's boundary, and here it survives in modest but legible form on the southern side, its base less than a metre wide and its depth around 30 centimetres. A hedge now runs along much of the perimeter, which gives the site a quietly agricultural appearance that belies its age.

This is a rath, the most common type of early medieval settlement monument in Ireland, typically consisting of a circular area enclosed by one or more earthen banks and ditches, used as a farmstead or high-status residence somewhere between roughly the fifth and twelfth centuries. The Shanmullagh example has been documented repeatedly across the cartographic record. It appears on McCrea's map of County Monaghan, produced in 1793, and again on the Ordnance Survey six-inch maps of 1834 and 1907, which means it was a recognisable feature of the landscape through the entire period of systematic Irish mapping. That continuity of documentation, across three separate surveys spanning more than a century, suggests the earthwork remained visible and distinct enough to be worth recording each time. A modern ramp entrance has been added at the southern side, indicating the enclosure has been incorporated into working farmland at some point in the more recent past.