It is likely that Roman expansion north of the Alps, and the victories of Julius Caesar in particular, encouraged Gaulish chieftains to lead their peoples into Britain and Ireland. There is some evidence that people of Celtic culture came from Scotland, and others from Iberia.
The last to arrive in Ireland were the Gaels, whose language was the basis for the variations of the Goidolic language(s) spoken in Ireland and Scotland (as opposed to the Brythonic origins of Welsh, Cornish and Breton). The Gaels were probably a ruling caste, overwhelming the indigenous peoples with superior weapons and chariot attacks.
Munster came to be dominated by the Eoghanachta [the followers of the god Eoghan], who remained in power for centuries. The Connachta [the followers of the god Conn] established themselves in the province that still bears their name. Their advance against the Ulaidh rulers of Ulster was slow, but the fall of Emain Macha to the Connachta in 450 AD marked the end of an era.
No real attempt was ever made to unify the island of Ireland politically, except for a certain grudging recognition of the ceremonial title of Ard Rí [High King], who traditionally dwelt at Tara, and probably exercised a pagan religious function, perhaps involving ritual sacrifice.
Irish Gaelic culture recognised no “supreme deity”, but revered Dana, the primary Mother Earth goddess, Lugh, a sun god of the harvest and of light; and An Dagda, the “good god” associated with very basic, earthy functions like eating, drinking and sex, as well as other gods. Druids made incantations at sacred oak groves, and precious objects were cast into sacred pools (later holy wells). The Gaels probably also revered the megaliths and holy places established by previous cultures. The highlight of the religious calendar was the New Year at Samhain, nowadays called Hallowe’en.
Early Gaelic culture involved a system whereby warriors gained honour by valour in battle. They rarely fought on horseback, but used chariots where the terrain permitted. They also hunted with hounds. It was almost invariably the aristocracy who did the fighting, while the peasantry carried out the everyday chores of farming. The main social unit was the tuath – the clan or sept, and the land it occupied, No coins were used, and the cow was the unit of exchange and measure of wealth. The whole system was supported by Brehon Law, whereby most crimes were settled by fines related to the status of the victim. There was no death penalty as such, but blood feuds were institutionalised.
It has been suggested that ancient Ireland was the origin of the legend of Atlantis, referred to by Plato. Other Greek and Roman writers made several explicit references to Ireland, which Julius Caesar called Hibernia. It would appear that the Romans often thought about invading Ireland, but never did. However, there was undoubtedly commercial and other interaction between Roman Britain and Celtic Ireland
In the 2nd century AD the Alexandrian geographer, Ptolemy, produced the first map of Hibernia with identifiable features, including Emain Macha and the first known settlement on the site of Dublin, which he called Eblana, thought to have been a fortified Roman trading post. In Irish, this place was called Ath Cliath [the Ford of the Hurdles], a shallow crossing point on the River Liffey between the kingdoms of Meath and Leinster. This was the convergence point of five major roads that had been built across the country by 200 AD. The nearby anchorage was called Dubh Linn [Black Pool]. Unreliable later chronicles record that in 291 AD the local inhabitants won a military victory over the king of Leinster.
In the 4th century, a s Roman power waned, Irish ships raided the coasts of Gaul and Britain, and Irish colonists settled in Cornwall and Wales, where they had no lasting impact, and the Hebrides, where their culture took root. Classical and later continental writers used the term Scotti to describe Gaelic speakers from Hibernia and western Caledonia, whose language eventually became the dominant language in Scotland.
The only form of pre-Christian Irish writing was Ogham, a script apparently developed by the Druids from one of the classical alphabets .It is mainly found on gallán (standing stones used as grave or boundary markers) in Ireland, but also in western parts of Britain and outlying islands. Internal evidence suggests a central European origin. Similar markings from 500 BC, have been found in northern Spain, and comparable carvings found in West Virginia indicate that Irish or other Celtic-influenced sailors may have reached the New World, possibly as early as 100 BC.