400–800

The middle centuries of the first millennium AD marked great changes in Ireland.

Niall Noigiallach (died c.450/455) laid the basis for the Uí Néill dynasty's hegemony over much of western, northern and central Ireland. Politically, the former emphasis on tribal affiliation had been replaced by the 700s by that of patrilineal and dynastic background. Many formerly powerful kingdoms and peoples disappeared. Irish pirates struck all over the coast of western Britain in the same way that the Vikings would later attack Ireland. Some of these founded entirely new kingdoms in Pictland, Wales and Cornwall. The Attacotti of south Leinster may even have served in the Roman military in the mid-to-late 300s.^[6]

Perhaps it was some of the latter returning home as rich mercenaries, merchants, or slaves stolen from Britain or Gaul, that first brought the Christian faith to Ireland. Some early sources claim that there were missionaries active in southern Ireland long before St. Patrick. Whatever the route, and there were probably many, this new faith was to have the most profound effect on the Irish.

Tradition maintains that in AD 432, St. Patrick arrived on the island and, in the years that followed, worked to convert the Irish to Christianity. On the other hand, according to Prosper of Aquitaine, a contemporary chronicler, Palladius was sent to Ireland by the Pope in 431 as "first Bishop to the Irish believing in Christ", which demonstrates that there were already Christians living in Ireland. Palladius seems to have worked purely as Bishop to Irish Christians in the Leinster and Meath kingdoms, while Patrick — who may have arrived as late as 461 — worked first and foremost as a missionary to the Pagan Irish, converting in the more remote kingdoms located in Ulster and Connacht.

Patrick is traditionally credited with preserving the tribal and social patterns of the Irish, codifying their laws and changing only those that conflicted with Christian practices. He is also credited with introducing the Roman alphabet, which enabled Irish monks to preserve parts of the extensive Celtic oral literature. The historicity of these claims remains the subject of debate and there is no direct evidence linking Patrick with any of these accomplishments. The myth of Patrick, as scholars refer to it, was developed in the centuries after his death.^[7]

The druid tradition collapsed, first in the face of the spread of the new faith, and ultimately in the aftermath of famine and plagues due to the climate changes of 535–536. Irish scholars excelled in the study of Latin learning and Christian theology in the monasteries that flourished shortly thereafter. Missionaries from Ireland to England and Continental Europe spread news of the flowering of learning, and scholars from other nations came to Irish monasteries. The excellence and isolation of these monasteries helped preserve Latin learning during the Early Middle Ages. The period of Insular art, mainly in the fields of illuminated manuscripts, metalworking, and sculpture flourished and produced such treasures as the Book of Kells, the Ardagh Chalice, and the many carved stone crosses that dot the island. Insular style was to be a crucial ingredient in the formation of the Romanesque and Gothic styles throughout Western Europe. Sites dating to this period include clochans, ringforts and promontory forts.

The first English involvement in Ireland took place in this period. In 684 AD an English expeditionary force sent by Northumbrian King Ecgfrith invaded Ireland in the summer of that year. The English forces managed to seize a number of captives and booty, but they apparently did not stay in Ireland for long. The next English involvement in Ireland would take place a little less than half a millennium later in 1169 AD when the Normans invaded the country.