Sebbi,
the celts were actually labeled as such because of their linguistic similarities. you are right though there was not one single Celtic "Rule" however; All over the Celtic lands that were guided by the wisdom of Druids would have had a similar outline as they (the Druids) were all taught the same way. the main differences would have been mostly linguistic and climatic.
it is an unfortunate situation that much of the Celtic beleifs have been lost to us but Celtism has evolved with the times and "Celtic" is no longer a linguistic description but has now changed to mean a genetic and even spiritual heritage.
Most of what we now about the Celts spirituality is based on the stories from Ireland and Wales. it is the messages that are found within which gives us the types of Celtic spirituality that we use today as well as the words of the Roman writers and the educated guesses of todays Archeologists, as such we can only assume that the beliefs and practices were only mildly different from tribe to tribe, just as it is in todays world (No 2 celtic Groups will do the exact same thing).
Even though the tribes were not united as a common body, there are many things that would indicate that there most definatley was a structure of some sort. for instance, there were rules of hospitality that each tribe and family had to uphold even if it was a visitor from a warring tribe. If the Tribes were so different from each other then the acts of hospitality would not have existed outside of ones own tribe, but since it was a social structure it was almost an executable crime (had it been a Druid at least) had the rites of hospitality not been afforded to the guest (this just one example, i can give more if requested).
I'm sure that each belief structure cited so far has its historical backing as having existed in it's own right, but to say that all of the celtic world revered, for example, Brigid seems absurb.
as for your comments about Brighid, there is plenty of evidence to say that Brighid was worshipped in EVERY Celtic speaking country. known by names with slight variances and given slightly varying attributes (as was necessary to those particular tribes). her name has been given to many places throughout the western end of Europe and was so highly honoured in Ireland that she was transformed into a saint. Bride, Brigantia, Brigid, Brigindu, Brid, Brig, Brigindo are just a few of the names this Goddess known by.
so....it is my opinion that the idea isnt as absurd as you think.
i hope i have answered you query.
Mac Sionnach