hello! just to check in: the religion is new. this is not a matter of being anti-minority religions (believe in whatever god(s) you want, or don’t) but simply a matter of historical accuracy.
for starters, Ireland had already been Christian for centuries before the Anglo-Norman invasion in the 12th century — the earliest Irish bishop was ordained in the early 5th century, the high kings stopped practicing pagan rituals in the mid-6th, and the Irish were enthusiastic missionaries throughout Europe by the 7th. the idea that the colonization of Ireland is in any way connected to the decline of pre-Christian Irish religion is entirely ahistorical nonsense. the Irish have been Christian for longer than the English (and the Welsh even longer than that). god knows (lol) I hate Christianity, but this is not a case of colonial imposition but rather of more or less enthusiastic proactive local adoption. (and in b4: the concept of “Celtic Christianity” as meaningfully distinct from Roman Catholicism is also essentially a modern invention; the actual disputes between the church in Ireland, Scotland, and Wales and the church in Rome were over things like the calculation of the date of Easter and monastic tonsure.)
we know very little about the pre-Christian religions (plural! if there was ever a single unified “Celtic” religion, we have no evidence of it) of Celtic-language cultures, and what we do know is largely filtered through heavily biased sources: Greek and Roman writers with an ideological investment in justifying imperial conquest and Gaelic- and Welsh-speaking Christian writers, mostly monks, writing anywhere from several centuries to a millennium or more after the arrival of Christianity. as such, it is very difficult — or, rather, impossible — to know to what extent the portrayal of (for example) the Tuatha Dé Danann in Cath Maige Tuired is actually representative of pre-Christian beliefs. it is likely that there is some pre-Christian basis to at least some medieval texts (though others may be entirely literary compositions), but at the end of the day all of the texts we have are Christian texts.
even notionally “reconstructionist” neopaganisms as a rule rely heavily on practices of selective interpretation, historical bracketing (e.g., of source texts’ Christian authorship), and romanticism, with a healthy dose of outdated and often colonial 19th-century philological and anthropological scholarship to round things out (Frazer…). the modern neopagan conception of druids, for example, owes an enormous amount to early modern Protestant writers who developed a revisionist conception of druids as prefigurations of the reformation (as practitioners of an entirely imagined “pure” proto-Christianity), and then of course to Iolo Morganwg’s 18th-century forgeries which invented an elaborate system of “druidic” beliefs partly based on these earlier writers and partly from whole cloth. (meanwhile, to my knowledge there are no references to druids in real medieval Welsh literature.)
the person in the tags is rightly criticizing the influence of the romantic nationalism of the 19th-century Celtic revival — of which Yeats, who was connected with early syncretistic occultist groups like the Hermetic Order of tbe Golden Dawn, is probably the most prominent example — on modern “Celtic” neopaganisms for contributing directly to the kind of romanticized conception of Ireland (and other “Celtic” regions) that I criticized in the first post.
all of which is to say, believe what you want, but don’t present these beliefs as unquestionably historical or especially as representative of “pre-colonial” religion, because they simply are not either.