Known as the Keltoi to the Greeks, and the Galli to the Romans, the peoples north of the Alps were of little initial interest in the ancient world. Then in the fourth century BC the great Celtic migrations from Central Europe pushed out in earnest, and overran the illustrious ancient civilisations on the mediterranean. In 390 BC the Celts became the only ancient civiliation to ever capture Rome; a century later they sacked prestigious Delphi of the Greeks. The Celts had arrived in the history books.
Unfortunately, there is little we can guage of their history from their own perspective, but we have many clues, through the historical sources and especially archaeological excavations. For a start, the Celts were a mixture of over 40 tribes who generally shared customs and beliefs, who pushed from central Europe and north into Britain, west into Spain, south into Turkey, but came up against the ever more closely migrating Germanic peoples in the West. Eventually Celtica became trapped in the vice of Roman imperialism and Germanic expansion, and gave way to one and then the other over time.
The Celtic peoples became absorbed, first by Rome, then by the Germans. But it was the literate Christian monks who followed again from Rome, after the Germanic conquests, who eventually began to take enough of an interest in recording local ideas. And so in the last quarters of the remaining Celtic languages, and over the following centuries, some small part of their rich tradition was finally put into writing.