Norwegian Vikings invaded the territory around Dublin in the 9th century, establishing the Norse Kingdom of Dublin, the earliest and longest-lasting Norse kingdom in Ireland. Its territory corresponded to most of present-day County Dublin. The Norse referred to the kingdom as Dyflin, which is derived from Irish Dubh Linn, meaning "black pool". The first reference to the Vikings comes from the Annals of Ulster and the first entry for 841 CE reads: "Pagans still on Lough Neagh". It is from this date onward that historians get references to ship fortresses or longphorts being established in Ireland. It may be safe to assume that the Vikings first over-wintered in 840–841 CE. The actual location of the longphort of Dublin is still a hotly debated issue. Norse rulers of Dublin were often co-kings, and occasionally also Kings of Jórvík in what is now Yorkshire. Under their rule, Dublin became the biggest slave port in Western Europe.

Over time, the settlers in Dublin became increasingly Gaelicized. They began to exhibit a great deal of Gaelic and Norse cultural syncretism, and are often referred to as Norse-Gaels.

The extent of the kingdom varied, but in peaceful times it extended roughly as far as Wicklow (Wykinglo) in the south, Glen Ding near Blessington, Leixlip (Lax Hlaup) west of Dublin, and Skerries, Dublin (Skere) to the north. The Fingal area north of Dublin was named after the Norse who lived there.

In 988, Máel Sechnaill mac Domnaill led the initial Gaelic conquest of Dublin. As a result, the founding of Dublin is counted by some from the year 988, although a village had existed on the site of Dublin since before the Roman occupation of Great Britain nearly a thousand years earlier.

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