The earliest mention of Dublin dates back to around AD 140. The Greek astronomer Ptolemy referred to Dublin as Eblana Civitas. Other 2nd Century sources site it as Elbana. However, the official date of the establishment of Dublin rests at AD 988, when Norse king Glun Iarainn recognised Mael II Mor as king of Ireland and agreed to accept the Brehon Laws and pay taxes.

In the early 10th Century, a Viking and a Celtic settlement were close to where Dublin is located 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 (the name of Dublin in Irish), used even today. The Vikings, or 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. Slaves were imprisoned and later sold by Norse and Irish chieftains. When the Brehon Laws were adopted, 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 actually 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. It even replaced Tara in Meath, formerly the seat of the Irish kings. Over time, the conquerors were assimilated into Irish culture and learned the Irish language and customs. Only a 20-mile area around Dublin, the Pale, remained under English rule and was fortified by the English against Gaelic attacks. Outside this area people were considered savages, hence the expression Beyond the Pale.