At the beginning of history, around the middle of the first millennium AD [notre emphase], the country was wholly Celtic in its language and its institutions. For linguists, this can only have come about by means of a significant immigration of Celtic-speaking people at some time in later prehistory. Such an intrusion is not, however, reflected in the archaeological evidence. There is thus seeming conflict between the two disciplines.
There is little to suggest that the earliest phase of the Irish Iron Age may be regarded as “ Celtic ”, however that term is applied. The Hallstatt culture is represented in Ireland by little more than a scatter of insular variants of the continental Gündlingen-type sword, a handful of winged chapes and a few other items […]. None of these objects is iron with the rather doubtful exception of a corroded and fragmentary sword blade from the river Shannon at Athlone for which a Hallstatt date has been claimed […].
Raftery, Barry (1996 ). « Ireland. A world without the Romans », dans Miranda J. Green (dir.), The Celtic World, ch. 33, Routledge, Londres et New York, p. 637.