There are some reasons, for linguistic and cultural reasons, where I would say this might be accurate:
- linguistically, ancient Irish was a Q-Celtic language, but from what is known, it must have been considerably closer to Gaulish than to Celtiberian.
- as I stated before, the Celtization of the British Isles must have happened relatively early, primarily because of the absence of non-Indo-European (or, indeed, altogether non-Celtic) languages, so in my opinion, the British Isles must have been settled by Celtic-speaking peoples relatively early. In my opinion, the distribution of R1b-L21 largely coincides with the Atlantic-Bronze Age in it's pattern - the northern part at least - mirroring the distribution of R1b-U152 which in turn matches Urnfield in it's distribution. Apparently, the subclade of R1b-L21, R1b-M222, appears to have originated in Ireland (the question when this happened might be interesting), and reached Scotland via the migration of the Scoti in the early medieval Ages.
- many of the Irish mythical figures are reflexes of not only Pan-Celtic but in particular also Gaulish deities (for example: Brighid - Brigantia, Goibniu - Gobannos, Lugh - Lugus, Ogma - Ogmios, Tuireann - Taranis). In contrast, none of the Gallaecian-Lusitanian deities was worshipped in Ireland.