Scholars are still debating whether the Celts introduced iron technology when they arrived in Ireland as invaders or settlers - or, whether their 'arrival' represented anything more than the development in Ireland of a culture that had already spread across northern Europe from Britain to the Balkans.
The general view is that the Celts moved into Ireland at some time after 500 BC, possibly in several waves, and that they brought with them the Gaelic language, a social order based on warrier values, and a religion involving features such as sacred groves, druids (priests), a severed-head cult and human sacrifice.
In view of their later history, it seems almost certain that they set up dozens of petty kingdoms, or 'tuatha' and fought endlessly among themselves; if the great cycles of myths are to be believed, Celtic pride, honour and passion were regular sources of epic violent destruction or confusion. Archaeological excavations confirm the widespread use of hill forts, island strongholds and other means of defence, although similar evidence of Iron Age insecurity is also found in other lands. Celtic warfare was a serious business, to judge by the number and character of Irish defensive arrangements. Even the tiny infertile Aran Islands were fortified as if in preparation for life and death struggles. In fact Dun Aengus on Inishmore, in one of the most impressive all all Iron Age forts, with three lines of ramparts backing on to sheer cliffs standing above the sea. Over the centuries there was a tendency for petty kings to become subject to provincial overlords, and by about AD 1000 the division of Ireland into the Five Fifths (Ulster, Meath, Leinster, Munster and Connaught) were most likely well established. From the 8th century AD one or another of these provincial kings claimed to be 'high king' of Ireland, but the title was rarely accepted everywhere and carried more prestige than power.
Celtic metalwork was executed with extraordinary skill, in a style typified by the flowing yet controlled lines of a Bronze Disc from Ireland. Later metalworkers have often paid the Celts the ultimate compliment of imitating their designs.
During the 1st century AD, the Romans conquered most of the Celtic Britian (present -day England and Wales, and southern Scotland). Until recently it was believed that Ireland remained undisturbed, but excavations in the 1990s uncovered a large fort of the Roman type on the coast of Drumanagh, north of Dublin. The proper meaning of the finds is still a matter of controversy, but at the very least they suggest that Roman influence on Celtic Ireland was more substantial than has sometimes been admitted.
Coins from the Drumanagh site suggest that it dates from the 2nd century AD, when the Roman Empire was at the height of its power and prosperity under the Antonines. Within a century, Rome was facing difficulties and Latin writers were describing the increasingly fequent raids launched by the warlike Irish on the coasts' of Britain. In time the Irish established colonies in Wales, Devon and Cornwall, and in the 5th century the Irish of Dal Riata in the north-east began to cross into Argyll; these invaders would eventually absorb the native Picts and found the kingdom of Scotland.
It was the early in the 5th century that the last Roman legions left Britain. The Empire rapidly disintegrated all over the West, to be replaced by new barbarian kingdoms