The difficulty for verbs conjugation and the original solutions of Gaelic, “creole-like” in some way, and its phonetic evolution push me to think it could be born of an ancient enough evolution on a lingua franca of already Celtic origin, In a network spanning central and northwestern Atlantic shores at Atlantic BA (1300/700 BC, after a previous time of genesis begun perhaps earlier). A Y-R1b-L21 dominated pop of BB origin been Celtized one time on the continent, before to be over-Celtized later at IA on the continent? I don’t think the heavy enough presence of Y-R-L21 in northern Spain even in Basque country, is completely independent of the BA. The well formed following Atlantic BA could have involved them more than Y-R-DF29, and the continental Y-R-L21, already of a more or less evolved IE dialect, could have shifted to an imperfectly assimilated Celtic dialect by contact(s) with “cousins” of eastern Gaul/Baviera. That said Y-R-DF27 shows also, outside Iberia, an Atlantic gradian along shores until Britain and northern Europe, spread opposite to Y-R-L21 and explained by the mutual exchanges at those times? Ireland didn’t receive too much people from the continent after BA. But that says nothing more concerning language.
So why not a BA genesis on the continent for a first wave towards West with substratum imput even before reaching Ireland – the western coastal regions of Gauls seemed lately concerned by the west-central Europe developments and novelties, after the first input of BB, more cultural than demic in western Gaul. More “autochtonous” Neolithic groups? Or Ireland has had its own substratum input on a lingua franca shared with Atlantic coasts without arrival of already ‘Gaelic’ speakers formed outside? the concept of 'lingua franca' for this supposed first Celtic input is maybe too "convenient" without any support?
In Ireland IA sent groups too, but not by force Gaelic speaking. There has been also P-Celtic groups even if scarce, and possibly even Belgae sets of clans. My preceding post about substrata could support an independent and maybe preceding colonization of not only Ireland but also Britain by Gaelic speakers, roughly Celtized.
AuDNA would show a continental input in southern Britain in its LBA/EIA period, but the demic one seems rather light then and the Y-R1b-U152 elements are scarce enough. The Qw- to P- shift could have needed some time before reaching Scotland, and at the demic level, Ireland had almost zero input for this period, at least at autosomals level. So, no important arrival of new tribes at IA into Ireland having sent DNA and language, on what we have todate. Because physical anthropology showed some ties between the IA elites of Ireland, Britain and N-E great Gaul, whose traces persisted in today populations of the Isles.
At the western continent level, Urnfields period could have redistributed pop’s here and there, pop’s which were there before, but not well mixed yet: a levelling in mixes, seemingly. Anthropology shows clearly moves of tribes, nevertheless. In Gaul around Paris and Elsass there has been some discontinuity in DNA at IA, according to the scarce surveys, and at anthropologic level Brittany (western Aremorica then) ‘alpinelike’ people appeared at late IA, from eastern Gaul, the most often females. It seems Celtic tribes or sets (sure naming then) by their historical numerous moves allover Gaul, have levelled progressively the DNA inherited from WHG (a lot “alpinized” in East), Cardial Neo, ‘Danubian’-neo and BB’s heirs, What did not erase completely the differences between Northern and Southern Gaul, but here we have non-Celtic ethnies at play too.
My purpose here is to say that a late Celtic Hallstatt then La Tène introgression could have celtized progressively Gaul, but that it cannot explain the Gaelicization of Ireland. There is no miraculous transmission of complete language without a demic input of some weight or some political superior organization. The first BB then Celtic (Gaelic?) inputS could have passed languages with, by time, some incorporation of substrata traits, trade only does not need an allover population shift of language, even more when we speak of non-centralized ethnies as BB’s and Celts. A second wave of IA Celts, spite not numerous, can pass its new dialectal traits on an already celtized pop, progressively here again.