The wall, still standing up to 1.5 metres on its exterior face and nearly 8 metres wide in places, encloses a roughly circular area measuring about 25.8 metres north to south. It is the kind of feature that reads as little more than a tumbled field boundary at first glance, yet its form, its scale, and its relationship to a ruined church at its eastern edge point to something considerably older and more deliberate.

Ecclesiastical enclosures of this type are the defining footprint of early Irish monasticism. Communities founded between roughly the sixth and ninth centuries were typically enclosed within a circular or oval boundary, known as a cashel or rath depending on its construction, which separated the sacred interior from the secular world outside. At Reentrusk, that logic is still legible in the landscape. The enclosing wall is truncated to the east by a later stone wall that abuts the north-east corner of the ruined church, suggesting the church itself may postdate the original enclosure, or at least that subsequent building altered the earlier arrangement. A gap to the west, where another wall crosses the line of the enclosure running from the church's north-west corner, complicates the picture further. The most plausible ancient entrance appears to be on the north-east side, where a radially set slab, placed on its edge in a manner typical of early ecclesiastical thresholds, is still visible. A line of stones just outside it may represent the remnants of an approaching trackway. Researchers including O'Brien in 1970 and O'Shea and Crowley in 1972 noted the site, with the latter recording traces of a possible souterrain entrance nearby. A souterrain is an underground stone-lined passage, commonly associated with early medieval settlements, used for storage or refuge. O'Donoghue, writing in 1986, described the place plainly as the site of an ancient monastery.