The Anglo-Irish Treaty of 1921 gave independence to 26 Irish counties, and allowed six, largely Protestant, Ulster counties the choice of opting out. Patrick was the son of a civil servant in Roman Britain, he was abducted and sold into slavery in County Antrim. Escaping to the European mainland, he eventually returned to Ireland as a missionary and bishop. His first church was at Saul, in County Down, and he later made Armagh the ecclesiastical capital of the island, as it has remained. Christianity provided not merely a religion but also a written language, Latin.
Canals connect the river with the Irish Sea at Dublin.
In the middle of the 9th century the Vikings began raiding the coasts of Ireland. They sailed up the Shannon Estuary, and in the year 922 established a colony on an island in the estuary. That colony was to grow into the city of Limerick. The great castle of King John in Limerick City dates from this period. It was not until 1210 that the County of Limerick was formally established.