The document summarizes the history of Dublin from its origins as a Viking settlement in the 9th century to the Norman invasion led by Strongbow in the 12th century. It discusses how Dublin was founded by Vikings and later became the capital of the British Empire in Ireland. It then covers Brian Boru driving out the Vikings, the clan wars between Irish chieftains, and Dermot McMurrough inviting Strongbow's Normans to help him, which led to Henry II conquering Ireland.