For Celtic culture, Ireland is much like Iceland was to the Norse. It was sufficiently removed from mainstream Europe to protect it from invasions and to isolate it from many of the cultural changes which wracked the face of early Europe. It allowed a singular perpetuation of pagan Celtic culture to fuse with Christian and the emerging European culture. This unique synthesis would provide the single most productive line of cultural transmission between Celtic culture and the European culture which grew out of classical and German sources.

Written history in Ireland began in the fifth century when Patrick came to Ireland and introduced literacy. Patrick came to the Celtic tribal kingdom of Tara, which was ruled by Leary, the son of Niall Noígallich. The sons of Niall ruled over two kingdoms in northern Ireland; these rulers formed a dynasty that would be called the Uí Néill; the south of Ireland was largely under the control of Munster. Patrick himself confined all of his activities to northern Ireland and the Uí Néill, particularly around the area of Armagh. Because he introduced the Irish to Christianity, European culture, and writing, he became the patron saint of Ireland.

In the 700's, Ireland became subject to Scandinavian raids and emigrations, just as most of the rest of Europe. The first to arrive were the Norwegians who attacked various islands and some of the headlands; in the 800's, however, the Norwegians began to attack the western coast of Ireland. In the mid-800's and all through the 900's, the Norse actively began to build fortified towns along the eastern coast of Ireland. In 841, they built the fortified town of Dublin (which the Irish called Ath Cliath, or, "the hurdle ford"), and would later establish fortifications at Cork, Waterford, and Wicklow, some of the central towns of later Irish history. Of these towns, however, Dublin was the center of all the Norse activity and served as their central base for raids all around Ireland and the Irish Sea.

The Irish at this time did not concentrate their population along the coast but lived inland the Irish also did not live in large and fortified towns. The introduction of both fortifications and something resembling urban life was originally introduced by the Norse.

Eventually, however, the Norse would come in conflict with the Danish and the area around Dublin became part of the Danish kingdom that had been established in northern England. The Irish, however, lived in individual tribal groups that were not united it wasn't until 1014 that Munster Irish under the leadership of Brian Bóruma defeated the Danish at Clontarf and finally expelled the Norse for good.

The Norwegians and the Danish, however, had largely stripped Irish culture of its greatest cultural artifacts. The only histories that were written of the Norse in Ireland were written by the Irish these historians were far from sympathetic to the invaders! Ireland, however, gained a fundamental shift in its cultural and economic practices. The Irish inherited from the Danes and Norwegians fortified coastal towns and a new economy based on trade and commerce with other Europeans. They also gave to the Irish more sophisticated skills in ship-building and travel.

The most important legacy that the Irish bequeathed to Europe was Irish Christianity. When Patrick came to Ireland in the fifth century, Christianity had spread across the face of Celtic culture but hadn't really penetrated the various Celtic cultures. It was spread very thin and practiced by a perishingly small minority in Gaul and Britain. It was also assuming a new, distinct character among the Celts, who combined Christianity not only with native Celtic institutions and religions, but with a plethora of eastern mystery religions. (Much of what we call modern "paganism" which points to Celtic sources actually originates in eastern, mystery religions that had been imported into Celtic culture.) It was this Celticized version of Christianity that Patrick brought with him to Ireland.