One more thing to consider, and getting back to the original point, it's not always useful to consider Classical sources from the first century alongside manuscript sources from twelfth century Ireland, and hold them to be of equal contextual value. Not that anyone has specifically done that here, but it's always something that clouds our thinking on this issue.
Not sure exactly what your point is? My view of the "Celts" is probably a very limited one of a culture which is much in decline by the mid 1st century AD in Europe and Asia, and by the end of the 1st century in Britannia. By the 2nd century it's all but gone, with the possible exception of Ireland. This idea my well be wrong, but aboriginal Celtic culture was certainly dead at this stage and replaced by a Romano Celt fusion - whatever that was.
So to gain an understanding of what Celtic culture was like one has to look at the references of the time, hence the classics. 10th century manuscripts are of little scientific value as the period in question is long passed, and the Celts left no written sources. Like I said at the start, the Romans differentiated the Celts from other cultures by simple criteria. Today we tend to employ more subtle criteria in our definitions, perhaps in an effort to try and justify 'modern' historical arguments about or heritage.
But maybe the Romans were right after all, and while we might like to consider the idea that the peoples in question were all linked by 'Celthood', maybe that's wrong, and while they may well have shared similar traits of one kind or another, they no more represent a Celtic nation that the aboriginal tribes of North America did.
All I can say it that based on Tacitus' definition of a German, I'm sure glad I'm not one
