County Derry from Samuel Lewis' Topographical Directory of Ireland 1837 County Derry is bounded by Co Antrim to the east, Lough Neagh to the southeast Co Tyrone to the south Co Donegal to the west and the sea Lough Foyle to the north. The name is derived from the Irish Doire which means an oak grove, the first settlements were set on what had probably been an island in the river Foyle. Traces of these early settlers have been found making Derry one of the earliest sites of human habitation in Ireland. Oak groves were sacred places for the Celtic peoples, not only of Ireland but also of Europe, and the early Christian evangelist's obviously realized this, using it to their advantage, by locating their churches and monasteries in or near such places.

In the sixth century saint Colmcille (Columba) (521-597) is said to have come from Donegal to escape the plague and founded the monastery of Derry, on land granted to him by a local king reputed to be his cousin the Prince of Aileach. Colmcille was responsible for founding many monasteries in Ireland and Britain, including Durrow in County Offaly and Iona on an island off the west of Scotland. It is thought that the first church was located on the site now occupied the little Church of Ireland Chapel of St Augustine. Sometime during the late middle ages the old monastery came to be used by an Augustinian congregation. The church of that monastery was used by the London colonists who came to build the walled city.

County Derry has an area of 2074 sq km (801 sq miles) the Sperrin Mountains are situated on its border with Co Tyrone. Its main rivers are the Foyle, Roe both of which discharge into Lough Foyle a tidal sea lough, and the Bann which forms th eastern border with Co Antrim, the latter reaches the sea at Coleraine.