Okay, I have to disagree with the idea that this genetics study has overturned previously held beliefs about the late prehistoric inhabitants of Ireland. It has not been the opinion of archaeologists studying the Irish Iron Age that there was a movement of people from Continental Europe to Ireland since the 1990’s (and you get archaeologists starting to express the idea as early as 1983 from my quick dive into my library) I go into the history if the idea here.

This understanding hasn’t percolated into popular culture and in fact the Celtomania of the 90’s has sort of set back popular understanding (in my view). There is a fantastic quote from J.R.R. Tolkien which sums up the word Celtic and how it gets used “To many, perhaps to most people outside the small company of great scholars, past and present, ‘Celtic’ of any sort is… a magic bag, into which anything may be put, and out of which almost anything may come… Anything is possible in the fabulous Celtic twilight, which is not so much a twilight of the gods as of reason.”

One of the big problems that non-specialists run into with trying to understand Iron Age Ireland (and Europe Northwest of the Alpine arc generally) around the time that classical Greek and Roman authors were describing the peoples that they encountered (roughly 700 BC to 400 AD, though later Roman sources refer to Celtic people in the past tense) is that two chronologically and geographically separate strands of evidence get married together: The aforementioned Greek and Roman writers such as Aristotle or Julius Caesar and the much later medieval Irish texts which only start to provide information about what they thought of their own origins in 7th to 8th centuries AD onwards (none of which ever uses the word Celt or Celtic). Though its not common now, a lot of popular writers in the 20th century would sprinkle in evidence from one source when describing the people depicted in the other. Its quite common for older descriptions (like *sigh* my primary school history textbook) of Pre-christian Ireland to blithely talk about Druids, a term Caesar uses for a religious specialist class he encountered in Gaul and Britain, or the predilection for going into battle stark naked bar sword and shield, something mentioned by, for example, Polybius, speaking about peoples from Northern Italy circa 200-100 BC. Creating a Pan-Celtic culture that spanned from Ireland to Anatolia (this is Tolkien's magic bag at work).

This view only started to be challenged, as I said, In the late 80’s and into the 90’s, with a massive backlash against the prevailing view, with some people going full Celtoskeptic and wanting to do away with the word totally, and on the other side (mostly in the Anglophone world) an accusation by some individuals that English archaeologists, being the colonialists that they were, were attempting to erase the historical differences of Scottish, Irish and Welsh peoples by denying them their history.

So, what were the beliefs of people in Ireland pre-christianity and how did they substantially differ from Continental or British peoples in the same period? The answer is that we cannot know for sure. There was a current in mid-20th century academia that saw the Irish mythological tales from the Medieval period as “A window on the Iron Age” (from an article of the same name from 1964) – which argued that the religion and social structure of pre-christian Ireland could be gleaned from the Medieval mythological tales such as the Táin Bó Cúailgne, once one stripped out the obviously fantastical elements. However, this rests on two key assumptions: That in the pre-christian period (which was without writing) there existed an oral tradition. That this oral tradition was then reproduced (centuries later) in the Christian period without substantial editing and christianisation (or that one could undo the editing and christianising).

This idea is now viewed with significant scepticism to put it mildly, a lot of the society and material culture described in the Irish mythological sources corresponds more so to the Medieval period than to the Iron Age. While there are some things that can be gleaned from linguistics (such as the correspondence of the Irish figure Lugh and the Gaulish Lugos), where the similarities begin and end is something that is still up for debate. Ireland is substantially different from Britain and the continent is significant ways during the Iron Age, but there are shared aspects as well. I hope this helps.

Sources.

Collis, J. 2003. The Celts, Origins, Myths, and Inventions

Mallory, J. P. 2013. The origins of the Irish

Raftery, B. 1984. La Tène in Ireland; problems of origin and chronology

Thurston, T. 2009. Unity and Diversity in the European Iron Age: Out of the Mists, Some Clarity? Journal of Archaeological Research 17(4).