Ireland mythology is full of "invasions" and even if distorded, I believe it proves ancient irish people had some "souvenirs" of past, mixed with some classical+christian "false corrections" -
anthropology (not simply linked to male HGs %s) show us a mix of ligneages which is not disproved by archeology nor history -
people were there before agricultors - agricultors came apparently through North : from where? maybe N-France, Brittany (some megalithical details: pottery and structure of megaliths) seem proving it) but that doesn' t contradict a remote ancient iberian origin : "iberian" can be mesolithic and neolithic in Ireland and what happened in Iberia happened also in France!!! (same different paleo-/meso-/neo-lithic men, same Celts, only proportions varied )-
La Tène seems more a cultural osmose than a colonisation there, but we have names given by the myth: Fir Galiain, Fir Bolg, Fir Domnain: they evocate gallic/brittonic and belgic celtic tribes, and with Ptolemeios we have tribes territories (the most in S and E) as those of Brigantes, Domnain, Menapii + maybe germanic or belgic-germanic: Cauci - datation? Bronze??? very possible -
so Y-R1b is the dominent male ligneage, OK, but surely by some bias - Bronze Age? at least, maybe older - were the Y-I2a2 and I2a1b in Ireland (more in North) before Y-R1b??? hard to say: but I could believe some BBs or Food Vessels were there before labelled Celts (or were Celts?); some exchanges took place long ago between Scotland and N-Ireland, on the two directions, inextricable (, first peasants, Beakers, Cruithni...) - before Celts, I wrote? but were all Y-R1b celtophone people?
the hardest thing (if not impossible!) is to put dates and order into the rubbish chronology and details of the mythology -
the phenotypes in ireland show ties with Mediterranea-Iberia, western and continental France and northern Europe, among them 'archaic' trends...