Most folks think Irish when they think Celts, and that's fine. Ireland, after all, is still the only Celtic country never conquered by Rome. When scholars and others speculate about Celtic society, they use records found in Ireland because those are the oldest available records about Celtic customs.
The earlier Celtic people of Ireland, Britain, the Iberian peninsula, France (Gaul), Switzerland, Eastern Europe, etc., did not write down their history. They considered history far too important to commit to writing. Instead, it was memorized by the educated classes--the druids--who could interpret it properly.
Buggers. Even that previous paragraph contains a ton of untestable assumptions--about the druids' roles, and the reasons that histories and beliefs were not written down, f'r instance.
Since there are some examples of these ancient tribes using Roman or Greek letters to note very mundane things--like accounting records or praises of deities--we know that the Celts used writing. Julius Caesar (not an unbiased source, IMHO) says the druids would not put their beliefs in writing for religious reasons. Unless we unearth a 2200-year-old history etching somewhere, I guess we have to settle for that explanation.
And that leaves the writings of Celtic Ireland, both pre and post Christian, to give us a clue to earlier Celtic beliefs. But the oldest existing records of ancient Irish history are still about seven centuries removed from the days of an independent, non-Romanized Gaul, and seven hundred years is A Long Time. Not to mention the geographic distance...
I'm playing devil's advocate here, but think about it. Seven hundred years ago, chances are the city/town you live in did not exist. Maybe your country did not exist. Seven hundred years ago, Europe was still recovering from the Black Death that wiped out a third to a half of its population. The Inquisition was just ramping up, so free thought was not only outre but downright suicidal. Most people in Europe were illiterate peasants who lived on the verge of starvation. How much have we changed since then? An immeasurable amount.
What's the point? Just that it seems awfully tenuous to me to speculate about the B.C. Celts and their society based on early Irish texts, such as a law code (the Brehon) first codified in 438 A.D. I'm not sure how old the oldest examples of the law code are--certainly more recent than 438--but tradition says that the laws date back to the 8th century B.C.
Tradition is very unreliable. My own family tradition handed down the story for 2 or 3 generations that a great-great-great grandfather of ours was a judge in the old country. Guess what? Someone did some Real Research and found out the tradition was completely bogus. No judges, only farmers and seed merchants. Period. So if tradition can be fictionalized within sixty or seventy years in one family, what can happen over seven centuries?
OTOH...This was a society that trained men and women to memorize long histories, we are told. And if the histories were sacred to them, would anyone dare screw them up? But OTOH...even written histories get warped over time. Every re-writing changes something very subtly, doesn't it? Language changes, and those who study this claim that it changes at a measurable rate.
I got into an argument with an engineering type person a couple of months ago about that. He was irate that language changed. He felt that someone should stop it from changing. He truly believed that was possible, and he felt that due to a lack of conscientious effort on the part of English professors, he had been cheated out of his right to comprehend and enjoy Shakespeare and Chaucer in their original words.
I thought he was crazy, though I didn't say so. Still, he grew quite heated over his points, and I can only assume that most people don't bother arguing with him often.
I digress. But this is a blog; I'm allowed.