The Irish and Scottish dynasties of Ui Neill descended from Niall Noigiallach, a High King of Ireland who died around AD 405. Politically, the former influence of tribal affiliation had been replaced by dynasties.

However, many kingdoms and people who were powerful in the past disappeared. Irish pirates raided the coast of western Britain in the same manner that Vikings would later plunder Ireland. Some of these invaders established new kingdoms in Pictland, Wales and Cornwall. It is said that when these invaders returned to Ireland as rich mercenaries, merchants, and having taken slaves from Britain and France (Gaul), they also introduced Christianity. However, there may have been missionaries active in southern Ireland even before St Patrick.
 
In AD 432, St Patrick arrived on the island and, in the ensuing years, worked to convert the Irish to Christianity. However, according to Prosper of Aquitaine, a historical chronicler, Palladius was sent to Ireland by the pope in AD 431 as the “first Bishop to the Irish believing in Christ,” which indicates that Christians were already living in Ireland. Palladius may have worked solely as bishop to Irish Christians in the Leinster and Meath kingdoms, while Patrick worked first as a missionary to the Pagan Irish, possibly around AD 461, then moving to more remote kingdoms located in Ulster and Connacht.

Patrick is said to have preserved the tribal and social patterns of the Irish, codifying their laws and changing only those that were in conflict with Christian practices. He was also the first to introduce the Roman alphabet to the Irish, which enabled the Irish monks to partially preserve Celtic literature.

The traditions of the druids, which represented the priestly and learned class of pagan Celtic society, shrunk and later disappeared, due to the spread of Christianity and ultimately in the aftermath of famine and plagues. Knowledge of Latin and Christian theology in the monasteries flourished shortly thereafter, and, as missionaries from Ireland to England and Continental Europe spread the news, scholars from other nations came to Irish monasteries. The isolation and extraordinary qualities of these monasteries helped preserve Latin learning during the early Middle Ages. Art flourished as well, in metalworks, sculpture and manuscripts, such as seen in one of the world’s most beautifully illuminated manuscripts of the four gospels ‘Book of Kells’, written around AD 800. Ornate jewellery was also prevalent, and many carved stone crosses spread across the island. Dry-stone huts, known as clochans, for singular dwelling, ringforts and promontory forts also peppered the island.