Described as early as 1950 as "easily overlooked", it is a nearly rectangular piece of schist, some 2.2 metres tall and 1.5 metres wide, incised with a cross whose ends flare outward and whose shaft curves and spreads in an unusual, almost organic way. Just to the left of the cross arm, a shallow square has been sunk into the stone. That small depression is easy to miss, but the whole composition likely carried serious legal weight for anyone who reached it in desperate circumstances.

Harold Leask, writing in his 1950 study of Glendalough's national monuments, interpreted the slab as a sanctuary marker. In early medieval Ireland, and continuing into the later medieval period, ecclesiastical sites could offer legal protection to those fleeing summary punishment. Reaching the boundary of such a sanctuary, and placing yourself under its sign, meant that violence against you was transformed from a private matter into a sacrilege. The Gateway itself formed the formal entrance to the monastic enclosure, making the western wall a threshold with real symbolic force. Leask also noted that a cross on the lintel of St. Mary's Church, positioned outside the enclosed area, may have indicated an outer boundary of the same privilege, suggesting that the zone of sanctuary extended in stages, with each carved marker defining a further degree of protection.

The slab sits in plain sight once you know to look for it, mounted at the western face of the Gateway arch. The incised lines are shallow and the stone weathers to a uniform grey, so the cross and its peculiar spreading shaft reward close inspection rather than a passing glance. The small sunken square beside the cross arm is easy to overlook unless you are specifically searching for it.