Running around its highest point are the remnants of a roughly oblong enclosure, about 154 metres east to west and up to 35 metres wide at its eastern end, defined by a bank of tumbled stone rubble that in places is nearly impossible to distinguish from the mountain itself. The wall blends so naturally into the contours of the summit that its outer slope merges with the natural break of slope, and much of it survives now only as a low, spread scatter of loose stone, five or six metres wide, with the eastern stretch reduced to little more than a scarp. Writing in 1839, a visitor named Ottway observed it as a low wall of large, uncemented stones that appeared to be of the most ancient construction, and the intervening two centuries have done nothing to sharpen the picture.

Excavations carried out in 1995 along the western and north-western sections of the wall uncovered an inner face built of flat schist stones, standing to about 0.6 metres, raised directly on top of natural scree. No outer wall face was found. More intriguing were the objects recovered from the shallow soils immediately in front of the wall: flint and chert flakes, several coloured glass beads, and two amber beads. The amber may belong to the Bronze Age, though the findspot was not a sealed or secure archaeological context. The blue glass beads are consistent with a date anywhere between the Iron Age and the early medieval period. None of the finds could settle the question of when the enclosure was first built. It may be prehistoric in origin, or it may have been constructed or adapted to define an early medieval monastic settlement, a possibility supported by what occupies the eastern half of the interior: the remains of an early medieval church and a penitential station known as Leaba Patrick, a name meaning Patrick's Bed. At the western end of the enclosure, a gap about two metres wide opens in the direction of Roilig Mhuire, a place name suggesting a burial ground, located some 450 metres downslope to the south-west and associated with a cluster of penitential cairns.

Anyone who climbs Croagh Patrick on the traditional pilgrim path will pass through or near this structure without necessarily realising it. The wall is most legible towards the western end, where the excavated sections give it a slightly more defined profile, but patience and a willingness to look at ground level rather than at the view are needed to trace even part of its circuit.