The earliest mention of Dublin dates back to around AD 140. The Egyptian-Greek astronomer Ptolemy noted a settlement called Elbana, which some claimed to be the modern day Dublin. However, the official date of the establishment of Dublin is AD 988, when Norse King Glun Iarainn acknowledged Mael II Mor as King of Ireland and agreed to accept the Brehon Laws and pay taxes.

In early 10th century, Viking and Celtic settlements were located close to where Dublin is today. The Viking settlement was named An Dubh Linn (Black Pool), referring to a nearby body of water. The Celtic settlement was called Áth Cliath (Dublin in Irish), a name that remains in used today. The Vikings, also known as Ostmen, ruled Dublin for almost 300 years until Irish King Brian Boru defeated them at the battle of Clontarf in 1014. During the Viking era, the city had a huge slave market where the traders were Norse and Irish chieftains. After the Brehon Laws were enforced, these practices were 'officially' abolished. However, the slave trade continued for another century.

The Brehon ('judge' in Old Irish) Laws governed everyday life and politics until the Norman conquest of Ireland in 1171. These laws were written in the period of Old Irish, between AD 600 and 900, and believed to reflect the traditional laws of pagan Ireland. After the adoption of Christianity, these laws existed for some time in parallel with Christian laws.

The Normans captured the southern part of Ireland in the 12th century, when Dublin came to the forefront of English power in Ireland. So much so, Dublin replaced Tara in Meath, the former seat of Irish kings. Over time, the conquerors were assimilated into Irish culture and adopted the Irish language and customs. Only a 20-mile radius around Dublin, the Pale remained under English rule and was fortified by the English against Gaelic attacks. Outside this area people were considered savages, and it is believed to be the origin of the expression “Beyond the Pale”.