It is pear-shaped in plan, measuring roughly 52 metres by 40 metres, and the interior is divided into a series of smaller enclosures by low revetment walls, narrow alleys barely a metre wide, and rubble mounds concealing souterrain-like creepways. A cashel, in general terms, is a stone-walled enclosure of early medieval date, but this one on Inishmurray is unusual even by the standards of the form: mural chambers are built into the fabric of the wall itself, corbelled and capped with massive lintels, and at least two low external openings, or creepways, allow a person to pass through the wall at ground level, the outer apertures standing less than a metre high. Two of what may once have been four such openings are now blocked from outside.

The monastery here is traditionally attributed to St. Molaise, though the date of its founding is unknown. By the eighth century it was clearly a functioning community of some significance: the death of one of its abbots was entered in the Annals of Ulster under the year 752, and in 795 the monastery appears in the Annals of Inishfallen as the target of a Viking raid. Inside the cashel walls stand three churches, among them St. Molaise's chapel and Templenatinny, as well as three clochans (small, beehive-shaped stone cells used by early monks) and four leachts, which are low, rectangular stone altars associated with devotional use. The cursing stones kept within the enclosure are perhaps the most arresting detail: flat, rounded stones used in a ritualised form of imprecation, turned against enemies by those with cause to invoke them. Cross-slabs, inscribed stones, and pillar stones were once numerous throughout the site; a number have since been moved to the island's former schoolhouse, now adapted by the OPW as a conservation storehouse. Beyond the cashel walls, clustered towards the southern shore, are further features of the monastic landscape, including a holy well, the Women's Church and its graveyard, a separate burial ground known as Relickoran, and the remains of a horizontal water mill. Around the island's periphery, isolated leachts served as stations in a turas, a devotional circuit undertaken by pilgrims on pattern day, the Feast of the Assumption on the fifteenth of August.

The cashel sits in the centre of the southern half of Inishmurray, roughly a hundred metres from the shore and about three hundred metres to the north-east of Clashymore Harbour, a natural rocky inlet. The island's nineteenth-century settlement, whose ruined stone houses run east to west along the southern shoreline, is woven into an older fabric: the drystone field system surrounding the cashel appears to overlie an even earlier arrangement of field boundaries, so that layers of habitation and use are stacked beneath and around the monument. The Board of Works carried out works on the cashel in 1880, and exactly what was rebuilt at that time remains unclear, which means some of what a visitor sees today may reflect Victorian intervention as much as early medieval construction.