Columba (Colum Cille, 'dove of the church') was born around 521 into the royal family of the northern Uí Néill in Donegal — a lineage that made him eligible for the Irish high kingship. He was educated at monastic schools under Finnian of Moville and Finnian of Clonard, and was ordained a priest. By his early forties he had founded several monasteries in Ireland, including Derry, Durrow, and Kells.

In 563, at the age of about forty-two, Columba left Ireland with twelve companions and established a monastery on the small island of Iona, granted to him by his kinsman King Conall of Dál Riata. The reasons for his departure are disputed: Adomnán's Life presents it as a pilgrimage 'for Christ,' but a persistent tradition links it to Columba's involvement in a dispute that led to the Battle of Cooldrevny (561), after which he was pressured or chose to exile himself in penance.

From Iona, Columba launched the mission to the Picts. His meeting with King Bridei at his fortress near Inverness — described vividly by Adomnán — was the critical breakthrough. The Pictish king allowed Columba to preach, and over the following decades, Christianity spread among the northern peoples. Iona also maintained close ties with the Irish churches and the kingdom of Dál Riata, and Columba played a significant diplomatic role among the Irish and Scottish kingdoms.

The monastery of Iona became a center of learning, manuscript production, and prayer. The Book of Kells may have been begun there. Columba himself was a poet and scribe — he was said to have loved books with an intensity that could get him into trouble — and his foundation continued to produce scholars and missionaries for centuries.

Columba died at Iona on the night of June 8-9, 597. Adomnán records that his last act was to bless the monastery's working horse, which had come to him weeping.

Columba became the subject of extensive hagiographic tradition. Adomnán's Life records numerous miracles: supernatural knowledge of distant events, power over demons, miraculous healings, prophetic visions, miraculous escapes. He is presented as a man of extraordinary spiritual authority, founder of a monastic empire, and missionary whose influence extended across Scotland, Ireland, and eventually English mission.