Celtic tribes have lived all over central Europe, from France to what is today Germany to Anatolia. The celts weren’t one, organized, ethnic tribe. They consisted of various different tribes, and some had cultural similarities indeed. The word “celt” stems from the greek “Keltoi”, which means as much as “brave”.
Unfortunately, we don’t have a lot of reliable sources today, most of it was uncovered by modern Archaeology, and some sources from the Romans. And the Romans did not have a neutral opinion on the various Celtic tribes either. In what is today France, the Gaul, in todays Switzerland the Helvetier, and so on. Since the attack on Rome they were seen as enemies of Civilization and shunned by the Romans. Julius Caesar said that the Celts and their Druids have an important function in the communities, they were priests, teachers and judges at the same time. A lot of the celtic tribes have lived between 1250 and 300, 800 A.D., in an area from Scotland to eastern France, Germany to Bohemia and further East. It is believed that they originated from Eurasia, possibly from India, around 3 000 years ago and traveled further north, possibly due to the climate.
The tribe of the Boii in what is today Bohemia traveled further down south to Italy. It is believed that the word Bayer, Bavarian, originates from the Boii tribe and their original settlement was in-between the Rhine and Danube. As germanic tribes such as the Teutonic travelled further down south, coming from the North, and the Romans travelled through the Alps, there were many conflicts between the celtic tribes with the germanic tribes and the romans, too. Around 220 A.D. the Romans have managed to wipe out celtic cultures of the Vendilici and other communities in the Alps. The area around the danube was less populated, although there were lager settlements around of what is today Kelheim and Wolfratshausen.
A lot of it has gotten lost, all we have left is Archaeology, a lot of artifacts and a few old settlement ruins all over Bavaria and the rest of Europe. We have sources from the Romans, but the truth is, much of it is still uncovered and is hidden, buried by earth and time.