According to early legends, Ireland was first inhabited by successive waves of invaders, the most important of which were the Nemedians, Fomorians, Fir Bolg, and Tuatha Dé Danann. These tribes are said to have been eventually subdued by Milesians (Gaels). The legends may have historical resonance in the arrival of Celts from continental Europe in the second half of the first millennium BC.

Ireland is mentioned under the name of Ierne in a Greek poem of the 5th century BC and by the names of Hibernia and Juverna by various classical writers. Little, however, is known with certainty of its inhabitants before the 4th century AD (Ireland was never part of the Roman Empire). At that time Irish tribes called the Scoti harried the Roman province of Britain. These expeditions were continued and extended to the coast of Gaul until the time of the Loigare, or King MacNeill (reigned 428-463), during whose reign St Patrick attempted to convert the natives.

Although Christianity had been previously introduced in some parts of Ireland, Patrick encountered great obstacles, and the new faith was not fully established in the island until a century after his death in about 461.

Medieval Ireland was a carefully stratified society, in which each individual had a social value measured in terms of honour. Kings, clerics, and poets had the greatest honour, with special status being given to craftsmen, musicians, and other skilled workers.

From early times each minor kingdom, or tuath, had its own king; these kings were subject to the ardrí, or high king, who usually resided at Tara, a hill in present-day County Meath. The laws were dispensed by professional jurists called brehons, who were endowed with lands and who were allowed important privileges.

In the 6th century extensive monasteries were founded in Ireland, in which religion and learning were zealously cultivated during the early Middle Ages of Europe.

From these establishments numerous missionaries, such as saints Columba, Columban, and Brendan, went forth during the succeeding centuries, while many students of distinction from Britain and the Continent visited Ireland to further their education. Seeking solitude, Irish hermits were also the first known visitors to the Faroe Islands, Iceland, and Greenland.

Irish monasteries were responsible for producing many great works of art, including high crosses; fine metalwork, such as the Tara Brooch and Ardagh Chalice; and illuminated manuscripts, such as the Book of Kells and the Book of Durrow.

Irish civilisation was heavily impacted by the incursions of the Scandinavians, which began towards the close of the 8th century and continued for more than two centuries.

From 795 the Vikings began raiding Irish monasteries for plunder and slaves. In the 840s they established raiding camps at Dublin, Cork, and elsewhere, which developed into permanent settlements and, later, trading centres. After a number of reverses at the hands of Gaelic chiefs, in the early 10th century a new wave of Scandinavians arrived from northern France, re-establishing old settlements and founding Limerick and Wexford.

The Vikings played a central role in Irish political and economic life until their signal overthrow at the Battle of Clontarf, near Dublin, in 1014, by the Irish king Brian Boru.