I could be (and probably am) totally wrong, But it's still interesting

From Pantheon.org--

In Irish-Celtic mythology, the Tuatha Dé Danann ("People of the goddess Danu") are the Irish race of gods, founded by the goddess Danu. These gods, who originally lived on 'the islands in the west', had perfected the use of magic. They traveled on a big cloud to the land that later would be called Ireland and settled there.

Shortly after their arrival they defeated the Firbolg at the first battle of Mag Tuireadh. In the second battle of Mag Tuireadh they fought and conquered the Fomorians, a race of giants who were the primordial inhabitants of Ireland. The Tuatha Dé dealt more subtly with the Fomorians than with the Firbolg, and gave them the province of Connacht. There was also some marrying between the two races.

The Tuatha Dé themselves were later driven to the underworld by the Milesians, the people of the fabulous spanish king Milesius.

Ok, in Greek mythology Uranus and Gaia gave birth to the Titans, who then defeated them. The Titans, like the Fomorians in the Celtic story, were a race of primordial giants. They were defeated by Zeus and the other Olympians, who then established themselves as the gods worthy of worship by humankind.

It's possible these stories are so strikingly similar because there are certain mythological patterns that people respond to and all that other Joseph Campbell shit, which I'm all for, but it seems just as likely that when the Romans came over and dominated Britain and Ireland, their stories conflated with the native ones and produced something which took elements from the pre-existing Celtic mythology and grafted it onto a Greek story.*

This happened more obviously in some cases (the Romans created a new goddess that combined one of their goddesses and the Celtic Brigid, called "Aquae-Sulis". You can see stuff deticated to her at the temple in Bath.).
There are of course other explanations--both mythologies feature a female triple goddess of war, but Pantheon also notes that the Celtic one, Morrigan, likely dates back to the "the megalithic [pre-historic] cult of the Mothers," so it's possible that they evolved from a common root.

But the theory doesn't seem implausible. It's also likely a fact in Native American flood stories, which are so similar to the one told iN Genesis that Western tampering seems a much more likely explanation than anything else.

Something to think about.

I wish I could take random classes in things that interest me, so I could write about shit like this and actually know what I was saying.

--
*Since the Romans' mythology came from the Greeks', obviously.