First, I am not sure we can assume a PIE expansion into Europe other than the original population.

The point is that the Atlantic Fringe and the Danube basin were in close cultural contact, seemingly always.

During the Urnfeld and Atlantic Bronze age we find a difference in burials but the religious artifacts and other practices, such as votive offering, hoardings and so on remained near identical. Hallstatt and La Téne cultures had a strong influence in both areas. In Classical Antiquity writers tell us that in this broad area a people they called Celts or Galls lived. They seemed to have a shared language and culture.

We can see differences in genetics, culture, language, and perhaps religion. We can also see similarities in these same areas.

What reason do we have to divide them? Why do we need to assume one group more pure than the other? Does it or should it even matter?

Your statement that you are not nationalistically motivated in dividing the Eastern Complex of Celts from the Western Complex, however, does show that you are aware of the origins of that theory and their nationalistic bent.

Whether you theorize them as Dacian or German makes no difference to the argument.

We have as many reasons to doubt the Celt were in Ireland as we do to doubt those in Bulgaria or Turkey. The only difference being that all the other Celts were wiped out or assimilated except in the far Northwest of Europe.

All areas warrant closer examination. General theories that promote one over the other is not helpful. Let us just see what future discoveries show us.