Historians: what kind of alcohol were the early Celts drinking?

By 'early Celts' I assume you mean Iron-Age continental Celtic speakers. Alcohol played an important role in the Gallic gift-giving economy, where powerful 'big men' gained influence and followers through their capacity to hold feasts and distribute Mediterranean trade-goods to members of their retinue. Barley-based beer called korma would have been mostly produced and consumed domestically and was considered inferior to wheat beer, called cervesa. Archaeological evidence indicates that Gallic beer was malted and probably included herbal additives like bog myrtle for flavouring.

According to Pliny the Elder, the Gauls brewed a large variety of beers including wheat beer, however he doesn't really go into detail about them. Diodorus, probably confusing mead for a kind of beer, claimed that they brewed beer with honeyed water although we do know that mead was produced and consumed in Gaul. However, it is entirely possible that some beer was brewed with honey.

Now, back to gift-giving: while the Gauls drank their domestically produced barley and wheat-beer, wine was consumed as an expensive trade good from the Mediterranean world. Wine would have been procured like other Mediterranean goods - powerful chieftains established strongholds alongside trade routes and traded raw materials like timber and minerals for them. One theory that explains the massive expansion of Celtic peoples around the 3d century BC into Northern Italy, the Balkans and Greece is that Celtic speakers were attempting to 'encircle' the Mediterranean world in order to keep reaping its benefits from a distance.

Diodorus Siculus wrote that the Gauls loved wine, and shockingly drank it in great quantities without diluting it. Archaeological evidence suggests that Diodorus was correct, as massive wine vessels like the Vix krater have been unearthed in Gallic graves. Wine would have been habitually enjoyed by the Gallic warrior-aristocracy, but was also likely consumed during communal feasts, provided by a 'big man' in the context of a gift-giving economy. Giving away such rare commodities as wine or jewelry ensured that the giver maintained the loyalty of his retinue, while potentially attracting more followers through news of his magnanimity.

So there you have it. Barley and wheat beer were the most commonly consumed alcoholic beverage, while wine was drunk as an exotic and expensive commodity that showcased the power and wealth of its buyer.