The valley of the river Foyle has been inhabited for thousands of years, with new evidence confirming the region's extensive history. In August 2000 an archaeological dig on the outskirts of Derry brought to light remnants of a farming community dating back to circa 4000 BC (a discovery that has great international as well as local significance). But if people had already settled then in the Foyle basin area, the date of the foundation of the city is traditionally set at 546AD, when St Columba (also known as Colmcille) founded a monastic settlement on an island in the river.

The locality was at that time thickly forested, and so the saint called his infant settlement Doire, meaning in Irish " a place of oaks ". The settlement quickly became known as Doire Colmcille. The monastery thrived from its inception, and very quickly a settled trading community sprang up around the monastic buildings. Derry was placed advantageously: the island in the river commanded the passage of the Foyle, at that time the main route into and out of central Ulster; it also stood at the meeting place of the lands of the O'Donnell clan of Donegal and the O'Neill clan of Tyrone, and the people of Derry were not slow to appreciate their settlement's strategic location.

Columba did not settle at Derry: he left the infant monastery for Scotland and went on to found the famous settlement at Iona, from where monks fanned out across Europe, spreading the Gospel as they went. He wrote of Derry: " The angels of God sang in the glades of Derry and every leaf held its angel ". The monastery was an important player in the federation of Columban monasteries that spread across Europe, and after it became an Augustinian congregation it maintained its important position.

We may surmise that the Vikings largely left Derry alone and the town continued to thrive into the middle ages, during which the Great Church (Tempull Mor) was built on the crown of the hill. At this point, Derry was an island no longer: the western arm of the river had dried up and the marshy area which remained became known as the Bogside. In the twelfth and thirteenth centuries AD, the town was in its heyday, but in the late middle ages, Derry began a slow decline and the ancient monastic buildings began to fall into disrepair. A 14th-century fire destroyed the great cathedral, of which no trace remains; at this point the town reached its nadir.

 http://worldfacts.us/UK-Londonderry.htm