Inishmurray may be named after the early saint, Muiredach mac Echdach (fl. early 6th century) of Killala. There are remains of an early Irish monastic settlement. Laisrén (Saint Molaise) Mac Decláin reputedly founded a monastery here in the 6th century. He was confessor of Saint Columba (Colmcille) after the Battle of Cúl Dreimhne on the mainland nearby.
The island's ecclesiastical settlement was attacked in 795 and again in 807 by the Vikings, and eventually the monks abandoned the island and it remained uninhabited until the first secular settlement, probably in the 12th century.
The enclosure wall is impressive - reaching 4.5 metres in height at its highest point and up to 3 metres thick. The site contains various ecclesiastical buildings including enclosures, a stone-roofed oratory, two churches, a clochán, a large beehive-shaped cell, a holy well and other remains including cross slabs suggesting foreign influences. The whole complex is composed of what is probably local sandstone rubble.
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The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.