Ran into this on the Tuatha de Danann and the 'invasions' of Ireland . . .
The Tuatha de Danann - as later recorded by Christian monks - were the pre-Christian deities of Ireland. It seems corresponding figures were worshiped across Britain and that the monks had a clear agenda when claiming invasions took place. I.e. painting themselves into the background.
Migrations and trade with Europe certainly took place, but the numbers and the nature of the finds are not indicative of Cortez-like invasions.
Rather than some sort of imported 'Celtic assembly' the Tuatha appear to have been the deified ancestors of the indigenous peoples of Britain and Ireland. Figures who are seen as arriving from the north/ four cities to the north of Ireland and very much associated with the mounds and cairns of the Neolithic and Bronze Ages.
The Tuatha seem to be pre-Celtic period (1st millennium BC) deities or otherworldly beings in terms of connections to the region and the earlier worship of ancestors. Significantly, core figures in Scotland, including Manannan, are 'bolted on' or complementary to mainstream Celtic mythology.
At which point things appear to get a bit complicated. In looking before 1,000 BC for high status groups/ structures we have the enigmatic Beaker People moving around between 2,800-1,800 BCE. (With a possibly Iberian point of origin). They are associated with copper trading/ metallurgy and present a discrete cultural pack, which operated alongside or merged into much of what was already in place.
While 'civilisers' and 'integrating' with Neolithic societies the Beaker People didn't build mounds so much as round barrow graves; didn't arrive from the north and weren't as obviously connected to earlier, ancestral worship within the region.
This may rise the possibility that the Tuatha recall or somehow echo some of the beliefs of the Neolithic. Archaeologically, we know that by broadly 3300BC groups from Orkney were visiting Ireland as the mounds later associated with the Tuatha were being developed in Scotland and Ireland. Plus, at the time, Orkney presents an urbanising, industrialising society at the level of classical Greece, but much earlier than Greece and after an extended period of development from 3,500BC.
This, perhaps, paints a picture of a sophisticated Neolithic culture becoming embedded in regional memory and feeding into the mythologies of later groupings. Except that it's an incredibly long time span.
Ness of Brodgar, with it's high painted walls and massive residences; allied to the trade in pottery and ideas that developed from there would likely have been been large in the minds of many in the Neolithic, but people stopped building mounds, some monuments were sealed off and burial and ceremonial practices adapted or changed. Consequently, we're left looking at myths that have definitiely been remythologised.
Perhaps the best we can say, until more archaeology helps out, is that groups of traders carried ideas and technologies back and forth across the whole region from as early as 4500BC. Along the way figures and cultures would have been remythologised - probably in terms of the distant past and links to more recent figures, e.g. popping a deity into your family tree shortly before a fondly remembered great grandfather has long been a means of trying to legitimise claims to land and title.
The Beaker People to some extent formed a high status aristocracy on the basis of their metallurgy - and some sort of reconciling of indigenous beliefs alongside any fresh inputs might reasonably be expected.
Overall, there is no material labeling of the term Tuatha alongside 'the original' mound builders and travellers of the Neolithic Highway between Orkney and Brittany; so no proof of the Tuatha corresponding to a particular group or culture.
In terms of any likely candidates/ the history of myth . . . the Tuatha have associations going back to the period of Brodgar, Newgrange and then Callanish. The Beaker People were sophisticated enough to seek advantage in connecting to awareness of earlier/ ancestral 'civilisers', but it appears an on-going, if evolving, series of indigenous cultures continued to pass ideas along while drawing on inputs from further afield.
We can be a bit more sure that the Beaker People were a lens through which the Neolithic world would have been filtered well in advance of the 'Celtic'/ Atlantic Bronze Age period of the 1st millennium BC. As a result, what we have of an account may be largely rooted in the blending of the Beaker People alongside the developing indigenous Neolithic and then Bronze Age populations.
A process perhaps outlined by the Mold Cape - which shows the goldworking skills, brought by the Beaker People, merged with numerous indigenous stylings - including much older design with a lengthy 'shelf life' from Scotland. Intriguingly this may have been a development of the earlier stomacher or breastplate, which much later again may have been used as a mark of status of some sort among the Picts.
The Mold gold cape. Bronze Age, about 1900-1600 BC/ From Mold, Flintshire, North Wales.
Image: Pictish 'Nobles'
A recent report on the Beaker People in Aberdeenshire may go some way towards explaining the 'echoing' as the old blends with the new, whether indigenous or arriving from Europe or Scandinavia. In Aberdeenshire we find a cultural pocket setting up its own type of monuments, while others continue getting on with making their preferred monuments nearby. Which appears to be quite consistent with the Beaker People's approach at Stonehenge, where they are believed to be responsible for the later stages when the Sarsen ring was added.
Significantly, whatever remains from the Neolithic and the early Atlantic Civilisation, the myth and archaeology becomes increasingly concerned with metals and later Celtic language and styling, which is rooted in the Bronze Age. So, all in all, we're clearly dealing with a very much layered history involving recasting Scottish culture time and again. However, the surge in ingenuity and invention at and connected to Ness of Brodgar certainly comes across and is increasingly evidenced as having persisted within Scotland and Britains' cultures.