[Gil]

The problem, as Milesian has said, is that "Celt" and "Celtic" should be used as an umbrella term that describes a cultural type that at some given point in the past covered most of Western Europe and more specifically the Atlantic Facade.

Instead of Celt/Celtic we should refer to both the Urnfield culture and the Hallstatt culture as the origin for the dispersion of the "celtic" culture. Regardless of genetic dominance of the R1b type in western europe, we know that the Halstatt culture came from central/eastern europe to western europe and I believe that, like had happened before, the local populations were neither displaced nor "erased" but simply assimilated in several degrees.

Now, in my opinion, the newcomers weren't different from the locals, meaning: the celtic tribes who slowly migrated into the west were not different from the indigenous tribes, more even because individual variation can sometimes occur without phenotype differentiation. Also, a "celtic" tribe might conquer a region and impose their domain and therefore transform the other culture also into "celtic" without modifying the original lineages.