Viking raiders appeared in the Shannon estuary in the early ninth century, when they sacked the important monsatery at Mungret. In 861 they established a longphort, or temporary ship-camp, on the banks of the river; this may ahve stood on the site of St Mary's Cathedral.
The foundation of the city of Limerick, however, is generally dated to 922, when the Viking leader Thormodu Helgason established a permanent base on King's Island, this is the area where King John's Castle now stands.
Long ships began raiding up the Shannon into the heartland of Ireland, looting monasteries and conquering the local Irish chieftains. Their settlement continued to thrive, soon becoming a significant trading port with links across the wide flung Viking world.