St. Columba, previously of Ireland, founded a Celtic monastery on Iona in 563. An educated nobleman and priest, Columba had founded several monasteries before running into some problems (the stories, not surprisingly, conflict with one another) in Ireland and departing for another coast. The Iona monastery gained prominence, sending missionary monks to Scotland, England, and the continent, and functioning as the center of the Irish church until a 664 schism. (The primary dispute had to do with the dating of Easter, as so many of these things do, and resulted in the decline of the Celtic church and the ascendancy of the Roman church in the British Isles.)
The Vikings showed up in the 700s, the 800s, and the 900s, destroying the monastery in 986. The famous Book of Kells illuminated manuscript was probably produced at Iona in the 800s (too late to have been written by Columba himself, as tradition long held) and removed to Ireland, where it remains at Trinity College Dublin, during the Viking raids. Iona itself would rise in another incarnation in the 1200s; in the meantime it would become the burial ground of kings from Scandanavia and Scotland, including Macbeth himself.