The site at Lisarrilly is a rath, an early medieval enclosed farmstead typically defined by one or more earthen banks and ditches, and here the circular enclosure measures roughly 33 metres across in both directions. What makes it quietly interesting is the gap between what survives and what was once clearly visible: the bank on the northern side still stands to an external height of over three metres, with an outer fosse or ditch cutting around the northwest to east arc, yet for much of the perimeter the earthwork has been so worn down as to be almost untraceable.

The site is old enough to have earned a place on McCrea's Map of County Monaghan, produced in 1793, where it appears as a recognisable feature in the landscape. Whoever mapped it then could presumably make out more of its outline than surveyors in later years. The original entrance has never been identified, and the interior holds a local tradition of a souterrain, an underground stone-lined passage or chamber often associated with early Irish raths and used variously for storage or refuge. No physical trace of one has been found, though such structures can be concealed beneath even well-examined ground. By 1995, a growth of blackthorn that had taken hold across the site had been cleared, which at least restored some legibility to the earthwork that remained.