The Vikings invade the territory around Dublin in the 9th century, establishing the Norse Kingdom of Dublin, the earliest and longest-lasting Norse kingdom in the British and Irish Isles, excepting the Kingdom of Mann and the Isles.

This corresponds to most of present-day County Dublin.

The Norse refer 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 reads: "Pagans still on Lough Neagh".

It is from this date onwards that we 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.

The actual location of the longphort of Dublin is still a hotly debated issue.

Norse rulers of Dublin are often co-kings, and occasionally also Kings of Jórvík in what is now Yorkshire.Over time, the rulers of Dublin become increasingly Gaelicized.

They begin to exhibit a great deal of Gaelic and Norse cultural syncretism, and are often referred to as Norse-Gaels.The extent of the kingdom varies, but in peaceful times it extends 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 is named after the Norse who lived there.In 988, Máel Sechnaill mac Domnaill leads the initial Irish 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.

Máel Sechnaill mac Domnaill is dethroned by Brian Boru (1002–1014).In the mid 11th century, the Kingdom of Leinster begins exerting influence over Dublin, but its kings remained Norse-Gaels until the Norman invasion of 1171.

Though the last Norse king of Dublin is killed by the Normans in 1171, the population of the city retains their distinctiveness based on their origins for some further generations.