A Beacon in the Dark Ages
In the latter days of the Roman Empire, it had become friendly to the Christian Church, but its fall certainly didn't mean the end of Christianity.
As barbarians invaded and occupied areas of the Roman Empire, Christians fought back, winning new lands and gaining new converts.
Invading tribes came from the north and from the east, lured by the riches of Rome. From Britain came the Picts and the Scots; from the Black Sea came Goths; from the north, Danes, Vandals, Angles, Lombards, Jutes, and Burgundians; and from the east Teutons and Huns. Each brought their own peculiar beliefs, some worshipping animals, others a multitude of gods.
Conversions in Ireland and Scotland
Christianity not only survived the onslaught, but fought back. Gregory the Great, pope from 590-604, set out to convert the newcomers, and earned his place in Christian history for his success in southern England.
Pope Gregory sent Augustine with 40 monks to England, winning over first the King of Kent, and then his people, establishing the center of Anglican Christianity, with its cathedral at Canterbury.
As with other parts of the Roman Empire, England had been partially converted under the Romans, although barbarian invaders had eradicated most outward signs of Christianity.
St. Patrick was not actually Irish. Born in Britain around AD 385, he was sold into slavery in Ireland. Later escaping to Europe, he studied at the Monastery of St. Martin of Tours, where he was ordained as a priest.
With missionary zeal, he returned to Ireland to undertake the conversion of the Celtic pagans and their Druid priests. Arriving in Ireland in AD 432, Patrick spent nearly 30 years establishing churches and monasteries on Druid sacred sites.
Following Patrick's conversion of Ireland, the Church turned its sight on Scotland. Born in Ireland, Columba was of royal blood, and may have been in line to become King of Ireland had he not chosen to become a priest. It seems likely he left Ireland as an act of penance, perhaps connected to a copy of the Gospels he had made, a dispute that led to a bloody battle.