Very little is known about the diet of the Celts outside of the very biased ancient sources. Here’s Strabo In Book IV, chapter 4 of “Geography.”
http://penelope.uchicago.edu/Thayer/e/roman/texts/strabo/4d*.html
Here’s an excellent article about the suspected pre-Roman Celtic diet:
https://storymaps.arcgis.com/stories/8ced9f818eb94929b90ce2e6c28dfda6
So what did they drink? Studies looking at the organic residues found on Greek ceramic fragments found in Celtic hillforts in pre-Roman France show that the Celts drank imported Greek wines, as well as local beer in these imported ceramic vessels.
https://amp.abc.net.au/article/11218012
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0218001
Max Miller does a wonderful job preparing a Celtic meal based on Strabo and the other ancient sources in this episode of “Eating History with Max Miller”: