Sorry. I can’t do the Celtic creation. It would have been great to do given the proximity to St. Patrick’s Day, but not a lot of their mythology actually survived. What are out there are recreations based on fragments or speculation. We just don’t have enough surviving text for a complete picture.
The reasons are complex in history, starting with the Roman incursions starting from the time of Julius Caesar. He conquered the Gaulic tribes easily enough, but never managed to quell Britannia. Each time he tried, he never managed to subdue the people, and the Romans were known for particular harshness with their enemies. Conquered territories such as Gaul were left relatively at peace so long as they didn’t have their culture destroyed repeatedly. The Romans conquered everything below Scotland, putting up Hadrian’s Wall for demarcation. So, for approximately 350 years, the Romans fought, on-and-off, with the Celtic tribes.
Christian assimilation came later, which would cause a corruption of the mythology in favor of the Christian mythos. But given that Christianization happened throughout Europe, and many mythologies survived—particularly the Norse which happened around the same time period—I think also that the Celts didn’t pass on their mythology in writing, primarily.
Ireland was largely untouched by the Romans, though there were trade relations, so the Romanization doesn’t account for the lack of mythology from those people. Most remaining evidence of mythology is archaeological rather than in texts. I believe that the archaeology was accompanied by oral tradition, and the Celts as a people simply didn’t embrace writing to the extent other cultures did. If anything, Roman conquest would have brought the strong tradition of writing to the tribes, but the insular Celtic tribes of Scotland and Ireland were never conquered. And because the Britons were so steadfast in the resistance to Roman conquest for so long, it’s not surprising that a degree of destruction accompanied it.
However, creation myths, particularly Continental European myths, have several patterns including polytheism, the presence of chaos/darkness/void, elemental interactions, construction out of a god’s or giant’s body, among other ideas.
And, interestingly enough, I have discovered a group who has sought to reconstruct the creation myth using fragmentary sources and these mythological patterns. So instead of my analysis of a myth, you can read an expert recreation of material most likely to be found in this Celtic Creation Myth.