Because the Celtic/Germanic tribes were illiterate, our knowledge of the ingredients and processes used to make central European Stone Age beers is sketchy. What we do know is that, until the modern globalization of trade, man has always brewed with the ingredients he found right at his door- (or cave-) step.
In antiquity, the bread beers of the Middle East were made mostly from emmer and barley. They were often flavored with dates and mandrake, a psychedelic plant that is mentioned in the Bible.
Ancient European beers, by contrast, were usually made from a combination of grains that included oats and rye. The European brews were flavored with such indigenous herbs and spices as juniper, caraway, bog myrtle, yarrow, wild hops, or mugwort. Both the mid-eastern and European brews made from bread were often spiked with honey.
Once the literate monks got into the act in the Dark Ages, our knowledge about their beer making is on very firm ground. They made beer the modern way. If we want to get an idea of the taste of these monastic brews, we can pursue several—albeit speculative!—paths.
Perhaps modern Belgian sour ales come closest to some of the Dark-Age brews. How about a Chimay Reserve (the one with the blue label)? Whenever a Dark-Age brew turned out particularly good, it might have tasted just a bit like the Chimay.
If you ever get to Finland, try a Lammin Sahti, a strong, juniper-flavored, low-carbonation ale made according to traditional brewing techniques that are probably 2,000 years old. (Sahti is not available outside its home turf.)
Considering that the old central European brews were all dark, fermented with any old airborne yeast, and made with just about any kind of grain, here is one very unorthodox but interesting way of, perhaps, replicating the flavor of a Dark-Age monastic brew: Mix half-and-half Samuel Smith Oatmeal Stout and Schneider or Ayinger Dunkelweizen.
This concoction has a few characteristics that should qualify it at least as a defensible attempt at imitating the original. Both brews are dark. The Samuel Smith brew has a slightly fruity, butterscotch-like component from the ale yeast strain used by that brewery, while the dunkelweizen has a few phenolic, clove-like components from its weissbier yeast strain. Both beers are ales—that is, they are made with top-fermenting yeast strains that are most likely to be dominant in open-fermented beers. And, the grain bills from the two beers together give us an authentic combination of flavor… from barley, wheat and oats.