The Iron Age was remembered in later literature as Ireland’s Celtic Heroic Age, the time of Cú Chulainn, Queen Maeve and the old Gods who would be remembered as the Tuatha De Danann.

From the 7th century BC onwards there is evidence of continuing contact with central Europe; including the civilisations which would become known as the Continental Celts. While it now appears unlikely that Celtic peoples ever actually invaded Ireland, both Ireland and Britain adopted a broad range of technologies and cultural practices shared by their European contemporaries. Among these was the adoption of iron working and perhaps the ancestor of the Irish language itself which shows a clear relationship to the Celtic Languages spoken in Britain and France. The most famous Iron Age monuments in the country include Rath na Rí on the Hill of Tara where a large earthen enclosure was erected at this period and its outer ditch filled with what appear to be the sacrificed remains of lifestock and some humans. Major tribal territories, perhaps the precursors of later kingdoms, were also carved out during this period with their borders defined by vast linear earthworks such as the Black Pig’s Dyke. Burial and settlement sites from this period have proven elusive but over the least fifteen years excavations along the routes of Ireland’s new road network have brought many new sites to light.

Between c. 100 B.C. and c. 300 A.D. pollen cores extracted from Irish bogs indicate a sudden and extensive decline in cereal cultivation, suggesting a large drop in population although this was swiftly followed by an equally rapid population expansion coupled with the adoption of new agricultural techniques such as dairy farming. Roman accounts show that Ireland was familiar to the Romans but in spite of a proposed invasion under Agricola, Ireland was never incorporated into the Roman Empire. New technology and cultural ideas seem to have been introduced from Roman Britain however, including the development of the earliest form of writing in Irish – the ogam alphabet which shows evidence of being based on Latin. The evidence for trade with, or raids on, Roman Britain is strongest in northern Leinster, with additional concentrations being known from north County Donegal and around Carlingford Lough. As one might expect, some of the largest concentrations come from the Boyne Valley area with Roman material being known from Tara, Newgrange and the great promontory fort of Drumanagh. The earliest form of Iron Age burial is the ring barrow, which tended to overlay unprotected cremations deposited in pits. Slightly later, the practice developed of crouched burials accompanied by small personal grave goods. These burials were often inserted into earlier mounds or monuments. The practice of supine (lying down) inhumation burial was increasingly adopted in Ireland in the fourth and fifth centuries and appears to have been adopted from Romano-British models and in the fifth and sixth centuries these burials were increasingly lined with stone in what are referred to as cist burials.

In the fourth and fifth centuries Irish raiders, referred to as Scotti or Attacotti, attacked Roman Britain, sometimes in concert with the Picts and Saxons. These Irish raids ultimately led to the establishment of Irish Kingdoms in Wales, Cornwall, the Isle of Man and Scotland, many of which persisted for centuries. The sixth century marked the high point of Gaelic culture in Britain, with the Irish language dominant in across much of the southwest and northwest. Gaelic would ultimately die out in Wales and Cornwall by the eighth or ninth centuries although it spread out across northern Britain, remaining the dominant language across much of what is now Scotland until the 12th century and surviving in areas of the highlands and islands until the present day.