Before excavation, the site at An Riasc, known also as Reask, offered little to the eye beyond a well-known cross-inscribed pillar stone, a second small cross-slab, and faint traces of some stone huts. Four other cross-inscribed stones associated with the site had been carted away during the nineteenth century. Field fences had gradually obscured the line of the original enclosure wall, and a roadway ran directly across part of it. What lay beneath was largely unsuspected.

Between 1972 and 1975, excavations directed by Thomas Fanning for the Office of Public Works revealed a site of considerable depth and variety. The roughly oval enclosure, measuring approximately 45 by 43 metres, was bounded by a cashel-type wall, that is, a substantial dry-stone boundary of the kind more commonly associated with early Irish stone ringforts, averaging 2.2 metres in width and surviving to a height of one metre in its best-preserved section. Inside, the highest ground to the east held a primary cemetery of over forty lintel graves, graves roofed with flat covering slabs, alongside a small slab-shrine and the remains of a dry-walled stone oratory built directly over the earlier burials. Scattered around the interior were the foundations of several clocháns, the corbelled beehive-shaped stone huts characteristic of early monastic settlements on the western seaboard, along with evidence of iron-working, bronze or glass-working, spindle whorls, quernstones, and a small corn-drying kiln outside the enclosure wall. A radiocarbon date from a hearth in the central area placed the earliest occupation at around 385 AD, and sherds of Late Roman amphorae suggested activity continuing into the seventh century. The main monastic phase, encompassing the oratory, the internal dividing wall, and much of the craft activity, may have extended from the eighth to the twelfth century. Unusually, despite its evident importance, Reask does not appear in any known historical record from the period.

After the ecclesiastical life of the site ended, the burial ground was repurposed as a ceallúnach, a clandestine burial place typically used for unbaptised infants and others excluded from consecrated ground, a practice that continued into modern times. The graves from this later phase were sometimes covered with quartz and sea pebbles, and among the objects recovered from them was a small figurine carved from soft stone showing an infant wrapped in swaddling clothes. The two conjoined clocháns in the northwest corner, which had once been connected by a paved path to the oratory, were later refurbished and used as animal shelters. The roadway that had partially buried one of the structures was diverted as part of the conservation programme that followed the excavations.