There is now broad consensus that there never was an ancient Celtic Ireland. The belief that Celts from continental Europe colonized Great Britain and Ireland in the last few centuries BC has not held up in the face of recent advances in archaeology, linguistics, and especially ancient DNA research. It is clear from the DNA record that Ireland was continuously inhabited by essentially the same people from the Bronze Age Bell Beaker folk (who arrived around 2400 BC) through the Iron Age and into historic times. The lack of any genetic evidence of an influx of Celts is supported by the absence of Celtic La Tene/Hallstatt artefacts in the archaeological record.
Nevertheless, there were major cultural changes in Ireland in the last few hundred years BC. Iron working had arrived by no later than 500 BC, accompanied by an apparent increase in prosperity and population size. A proto-Insular Celtic language took root at around the same timeat latest, by 300 BC. But rather than having been introduced by invading Celts, the Goidelic and Brythonic languages are now believed to have originated in the lingua franca of an Atlantic-facing trading network centered on Iberia.
To the extent that the question is asking what life was like in Iron Age Ireland, it should be kept in mind that the era spanned 1,000 years, from ca. 600/500 BC to 400/500 AD. Modern readers tend to compress long periods of olden times and assume that what was true at some point during those times was true for the duration. But 1,000 years is a very long timeas long as the period between the Battle of Clontarf and our dayand it is reasonable to expect that there were immense changes in Irish life over that long span.
The archaeological record shows that after the introduction of iron- making, Atlantic Europe recovered from a late Bronze Age economic collapse. Ireland was once again part of an Iberia-centered trade network, and no doubt those native chieftains who were the first to acquire iron weapons and weapons-making technology enjoyed a competitive advantage over neighbors who were slower off the mark. Artefacts from early in this period (before 150 BC) show strong stylistic influences from continental Europe; but after that there are more similarities with British styles and techniques. Some Irish weapons, jewelry, and other products from the later Iron Age are of exceptional quality.
Nothing lasts forever of course, and the pollen record indicates a sharp decline in human activity between about 100 BC and 300 AD. It is possible that this collapse in population and agricultural activity may be be attributable to climate change, i.e., a general and sustained decline in temperatures. After about 300 AD, the population rebounded, something that continued after the beginning of the Christian era.
Irish society was tribal during this period, with people owing their primary loyalty to their immediate kinship group and thereafter to a larger tribe headed by a king or chieftain. We can be a little more confident regarding what life was like in the latter part of the Iron Age because: (a) we have fragmentary accounts of Irish society from Greek and Roman travelers; (b) there was much continuity between Irish society before and after the introduction of Christianity, when we begin to have accounts by Irish writers; and (c) pre-Christian epics in the oral tradition like the Ulster and Fenian Cycles came to be written down (albeit with Christian interpolations), providing us with additional insights into life in the late Iron Age. What we see is a society in which a hereditary warrior class held sway, kings were elected from within the tribal ruling family, poets and smiths were among the most high-status professions (and hereditary), and cattle-raiding was endemic.
Perhaps the most characteristic structure associated with the Iron Age is the hillfort, a fortified refuge typically incorporating earthen ramparts and stone walls built on a hilltop with a commanding view of the surrounding countryside. Forts like this are common throughout much of Europe; some date back to the late Bronze Age and in Ireland they continued to be used into early historical times. While most Irish hillforts are between five to ten acres in area, a few are as large as 40 acres and functioned as important tribal centers. Some of the most importantsuch as Navan Fort (Eamhain Macha) in Ulster, the Tara ringfort in Meath, and Dún Áilinne in Leinsterappear ultimately to have become more ceremonial than defensive in their purpose.
Another feature of the Iron Age landscape was the defensive dyke or linear earthwork, some of which still survive, at least in partmost notably, the Black Pigs Dyke in Ulster and the Cliadh Dubh in Munster. These constructions, which may have served as boundaries between ancient kingdoms and impediments to cattle raiding, required a considerable social organization. For example, one of three surviving sections of the Cliadh Dubh stretches over twenty kilometers from the Ballyhouras to the Nagle Mountains. The Iron Age Irish also built impressive corduroy roads over long stretches of bog and wetland. Indeed, some parts of Ireland may have had better roads 2,000 years ago than theyve ever seen since.
Most people didnt live in hilltop forts of course. The typical Irish house was circular, with wickerwork walls and a thatched rooflike Bronze Age and early medieval homes in fact. Snug in such a house, a family might enjoy a meal of beans or nuts with cheese and barley bread, washed down with buttermilk, before settling down to an evening of story telling by the (smoky) fire, while rain on the wind watered the fields outside.