This development is, of course, not unconnected with the Pythagorean revival in philosophy. But by then the Pythagorean ban on eating meat had been reinforced by a philosophical tradition going back to Theophrastus and Xenocrates, the heads of Lyceum and Academy respectively in the late fourth century B.C. Theophrastus’ work On Piety (much of which has been preserved in Porphyry’s On Abstinence) presented a systematic argument against animal sacrifice, with an interesting theory on the prehistory of divine worship. Theophrastus cited Empedocles on the Golden Age (when Aphrodite ruled instead of Zeus, and “the altar was not drenched with the unspeakable slaughter of bulls”), and he claimed that the original offerings to the gods were made exclusively from the fruits of the earth.
Theophrastus emphasizes the anatomical and psyholocial features that we share with the animals, above all, sense perception and feeling. He thus offers, for the first time, a philosophical basis for the notion of a moral community between us and the animals. On this view, we have the right to kill dangerous animals, but only in the same way that we have a right to protect ourselves against criminal human beings. Xenocrates, on the other hand, was concerned not only to protect animals but also to preserve the human being from contamination: animal food will assimilate the eater to the souls of irrational beasts. In Xenocrates we recognize a forerunner of the NeoPythagorean asceticism. Xenocrates; successor Polemon defended a similar view, and abstinence from meat became characteristic of the Platonic school. The writing of Xenocrates and Polemon are lost, but the Platonist case for vegetarianism, on a wide variety of grounds, is abundantly preserved in the writings of Plutarch, notably On the Eating of Flesh, Whether Aquatic Animals are more Intelligent than Land Animals, and On the Use of Reason by Brutes
Diodorus Siculus reported that the Druids were “philosophers and theologians,” “skilled in the divine nature,” and able to communicate with the gods. Julius Caesar wrote that they had philosophical and religious beliefs pertaining to the “powers and spheres of action of the immortal gods”; that they had “much knowledge of the stars and their motion, of the size of the world and of the earth, of natural philosophy.”
Strabo and Cicero said the Druids had the knowledge of nature which the Greeks called physiologia. Other ancient writers linked the Druids with the Pythagoreans. Diodorus, Ammianus and Valerius Maximus associated the Druidic belief in immortality with the theory of metempsychosis, making Druids “members of the intimate fellowship of the Pythagorean faith.” Some went further and derived the Pythagorean school of philosophy from the Druids. Iamblichus, for example, maintained that Pythagoras was acquainted with the Celtic mysteries, a statement confirmed by Clement of Alexandria, who in about AD 200 wrote that philosophy had been studied by the Druids before the Greeks.
One of the few things that both the Greco-Roman and the vernacular Irish sources agree on about the druids is that they played an important part in pagan Celtic society. In his description, Julius Caesar claimed that they were one of the two most important social groups in the region (alongside the equites, or nobles) and were responsible for organizing worship and sacrifices, divination, and judicial procedure in Gaulish, British and Irish society.
Alexander Cornelius Polyhistor referred to the Druids as philosophers and called their doctrine of the immortality of the soul and reincarnation or metempsychosis “Pythagorean”:
Others have invoked common Indo-European parallels…
So if the Pythagoreans practiced vegetarianism + forbade animal sacrifice… and they learned from or were Druids… and the Druids kept the Lore / Law for all Celtic society I think it would be a reasonable assumption that the entire Celtic society in origin were vegetarian… and there are common Indo-European parallels
The Avesta of Zoroaster (Iran)
The Vedas of India + Thule…
The inhabitants or people of Thule are described in most detail by Strabo in his Geographica, having preserved fragments of the account of Pytheas who was an alleged eye-witness in the 4th century BC:
…the people (of Thule) live on millet and other herbs, and on fruits and roots; and where there are grain and honey, the people get their beverage, also, from them. As for the grain, he says, since they have no pure sunshine, they pound it out in large storehouses, after first gathering in the ears thither; for the threshing floors become useless because of this lack of sunshine and because of the rains.