All descriptions of druids were by Greek or Roman authors and essentially concerns ancient Gaul with possibly an indirect mention in Galatia (which would be attributable to its Gaulish population) : while they're still wrote about in the IVth century AD, it mostly accounts for a situation between the Vth century BCE and the Ist century AD. Mention of Irish druids are found in both medieval epics and legal tracts, written in the early medieval period by Christian monks.

Druids in Britain are furthermore poorly attested for : Caesar points that some Gaulish druids went there to further their studies, possibly because they had a “purer” teaching compared to late Gaulish druids (more on that later), but safe mentions such as in Tacitus’ Agricola, we know little about British druids that are, more by default than by evidence, more lumped together with Gaulish druids due to their probable disappearance in Antiquity. Disappointingly, any comparison would thus more lean on the similarities and the differences between ancient Gaulish druids and their early medieval Irish namesakes, (southern) British druids being there tentatively considered closer to the former due to the other cultural and social similarities between ancient Gaul and Britain and the Caesarian affirmation that both were connected : there’s nothing that proves that they were present or similar in Northern Britain either, tough.

The geographic and chronological distance of the sources are obvious, and these were written by people that belong to a more or less different mental universe than what they described or copied. It didn't discouraged comparison, or even undue anachronistic mix-up; but remains the question on how to undergo relevant parallels can be done, if at all. Are differences observable between Gaulish and Irish traditions a result of sources written by two different cultural mindset; or should we consider these sources as realistically describing contemporary situations (or using authors that did)?

Indeed, descriptions of Gaulish and Irish druids doesn't always match very well, while not necessarily radically different. A set of explanations had been proposed : considering Druids as an essential part of a "pan-Celtic" spirituality (in particular from an Indo-European perspective or even, speculatively, “Old European”); considering Druidism as an essentially Gaulish development out of local dynamics and Mediterranean influences; the possibility of "proto-Druids" (from pre-Indo-European or Proto-Indo-European features) that would have evolved apart among protohistorical peoples, without being noticeable before the late Iron Age, etc.

While both are described as having an high social position, the former seems to have fulfilled a much more institutional role than the latter : having a legal and juridical function, which included a diplomatic part (mentioned by Strabo as being able to stop wars and best illustrated by Diviciacos' embassy at Rome). Diviciacos, the only ancient druid we know by name, playing otherwise a main political role in Aedun politics^(1) without being magistrate ^(2) although he was possibly a fiscal public contractor, similar to a roman publicanus. Eventually, what might have been an important moment of a ill-defined Gaulish identity, similarly to Olympic Games to Greeks for example, was the annual assembly of Druids (modeled or modelling the "lay" regional assemblies) of all Gaul; an institutionalization that doesn't have an equivalent in sources about Irish druidic function, which if still benefiting from a certain sacrality of the Druid being above the king (and, in Keltikè, magistrates)^(3).

This institutional role might be what both propelled the expansion of druids in ancient Gaulish polities, but also a great pressure in the IInd and Ist centuries (which was a time of great and quick social changes, due to the Cimbric and Teutonic migrations from one hand; and the crushing Roman influence in trade, material culture and politics). Either a period of transformation or decline of druidic influence, it's possible that druids, through the trade and cultural proximity with Britain, turned it as a "druidic land". From there, did these druids had the same importance they had in Gaul? It's hard to be affirmative : the island of Mona could have been a locus consecratus comparable to Carnutes', the few we know of southern British society hints at further similarities with Belgica society (more socially and politically archaic compared to Keltikè's peoples, including Belgians) and a broad common cultural frame of reference (which could be found in ancient Ireland too, with some deity names and associations, notably Lugos/Lugh and the crow), but that remains speculative and poorly referenced by more or less stereotyped Roman sources.

If “ancient style” druids were to be found in Ireland, it would probably be in this broad end of the millennium: it’s not impossible that it would have been carried by an handful of Gaulish or British migrants in southern Ireland, although it’s similarly speculative and indirectly supported at best trough some similar features between southern Ireland, southern-western Britain and northern-western Gaul.