Challenges of Studying Celtic mythology
The pre-Christian Celts left no written records of their beliefs, so we must rely on the work of archaeologists to gain a picture of what Celtic religion was like in ancient times.

While the Celts did indeed have a writing system (manifest, for example, in the old Irish writing system Ogham), they valued oral tradition over written narratives.

We must also rely on the accounts of the Roman conquerors.
As we all know, history is written by the winners.
We may never know the Roman conquest from the Gauls' perspective, but we can be sure it must have been like the Asterix cartoons from France.

All the evidence we do have of Celtic stories and myths was written down after the Celts became Christian.

Nevertheless, the Medieval Irish and Welsh literary cycles offer us a glimpse into the world of the Celts.