The first known violent invasion of the Vikings took place in AD 795, when from Norway they raided the isle of Lambay, situated off the Dublin coast of Ireland. Early Viking incursions were basically small in scale, but fast. The invasions interrupted the Golden Age of Christian Irish culture with 200 years of constant warfare. Waves of Viking raiders ravaged monasteries and villages throughout Ireland. The larger part of the early invaders arrived from the inlets of western Norway.
By the beginning of AD 840, the Vikings began to establish settlements along the Irish coasts and established villages in Limerick, Waterford, Wexford, Cork, Arklow and most notably Dublin. Early to mid-840 documents show that the Vikings were moving further inland to loot, via such rivers as the Shannon, and then retreating to their coastal headquarters.
In AD 852, the Vikings’ Ivar Beinlaus and Olaf the White arrived in Dublin Bay and built a fortress, on which the city of An Dubh Linn, or Dublin (Black Pool), now stands. Olaf was heir to the Norwegian throne and proclaimed himself King of Dublin. After a few generations, a group of mixed Irish and Norse ethnics arose. The heirs of Beinlaus founded a long dynasty based in Dublin, which dominated large parts of central and eastern Ireland.
The Vikings never reached full domination of Ireland, but often fought against such Irish kings as Flann Sinna, Cerball mac Dúnlainge and Niall Glundub. In the end, the fate of the Vikings was sealed by the High King of Ireland Mael Sechnaill mac Domnaill of Meath in the AD 980 Battle of Tara.
Early Ireland had an extraordinary government. The country was divided into many small kingdoms, or tuaths. Each tuath’s king was voted by all the free men of its territory, the overall territory of the land properties of its members. About 80 to 100 tuatha existed together at any time throughout Ireland. Above the tuaths were larger provincial kingdoms, similar to municipalities.
By the end of the 12th Century, Ireland was separated into a hierarchy of smaller kingdoms and over-kingdoms. Force was exercised by the leading men of a few regional dynasties fighting against each other for supremacy over the whole territory of Ireland, one of which included the king of Leinster Diarmait Mac Murchada, anglicised as Diarmuid MacMorrough, who was exiled from his kingdom by the new High King Ruaidri mac Tairrdelbach Ua Conchobair. Fleeing to Aquitaine, Diarmait gained permission from Henry II to use Norman forces to re-establish his kingdom. The first Norman knight set foot in Ireland in the mid-12th Century, followed by the main forces of Normans, Welsh and Flemings in Wexford in 1169. Leinster was soon taken back, and Waterford and Dublin were under the control of Diarmait.