Born c. 484, Tralee, County Kerry, Ireland
Died c. 577

During the fifth century Christianity blossomed in Ireland. Until that time it was largely a pagan country, but through the missionary efforts of Saint Patrick—a British Christian who became a monk while in France—and other traveling priests, the island was converted. (Christianity had actually been introduced to the British Isles by the Romans centuries earlier.) Monasteries were founded across Ireland, and from there missionary groups were sent out to further spread the teachings of Christ and the Church of Rome. The first converts were in nearby territories, in places like England and Wales. But the Irish religious (members of a religious order, such as monks) also established Christian communities in France, Germany, and northern Italy. Many monasteries in Ireland kept records of these missionary ventures, giving us valuable information about early Irish explorers.

As...