THE CELTS
The origins of the people who became the Celts are lost in the mists of time, but the ancestors of the Celts arose like other Indo-European groups, among the central Eurasian steppes sometime in the Bronze Age. These tribes began to migrate to the west and occupied areas of present-day Europe and eventually also part of what is now Turkey. It is likely that the Celtic culture came to Ireland from Britain & elsewhere as a gradual trickle of newcomers, rather than a large invasion force, with their influence gradually spreading.
The 2nd century BCE saw the beginning of a sophisticated Iron Age culture within Ireland with the Celtic language achieving a dominant position. Along with a new language the Celtic culture also brought many beliefs & traditions however those of the older pre-Celtic people already here, seem to have been assimilated and ritual importance was still accorded to the ancient sites of the land.
We also have within Ireland the legacy of the Celtic goddess & gods which (arguably) includes Danu, Donn, Bóinn, the Daghda, Lugh & Brighid. The prominence of goddesses in Irish tradition is striking with the goddesses of territory / nature/ fertility being supreme, particularly the triadic goddesses Eriu, Banba & Fohdla, The triple Morrighan and the three Machas. The Goddess in Ireland encompasses fertility, sovereignty, war, sexuality, shape-shifting, maternity and old age.