If the descriptions of what they say that they did was undertaken earlier than the time of the Celtic influx, then their ways may have been incorporated into the Celtic tradition. If this happened, some form of their tradition may have existed during the Stonehenge era.
In the book 'Celt, Druid and Culdee' by Isabel Hill Elder I found this passage attributed to Ceasar:
'They hold aloof from war and do not pay war taxes; they are excused from military service and are exempt from all liabilities. Tempted by these great advantages, many young men assemble of their motion to receive their training, many are sent by parents and relatives. Report says that in the schools of the Druids they learn by heart a great number of verses, and therefore some persons remain twenty years under training. They do not think it proper to commit those utterances to writing and in their public and private accounts they make use of Greek characters. I believe they have adopted the practice for two reasons - that they do not wish the rule to become common property, nor those who learn the rule to rely on writing, and so neglect the cultivation of memory; and, in fact, it does usually happen that the assistance of writing tends to relax the diligence of the student and the action of memory ...They also lecture on the stars in their motion, the magnitude of the earth and its divisions, on natural history, on the power of government of God; and instruct the youth on these subjects.'