Since I'm not as familiar with the celtic mythos, I'm afraid I can't directly answer you. However, it sounds like your overview might be missing something, in the same way of saying "Kings 1 and 2 is just an extended family tree of Jewish kings."
Yes, it is. But the ultimate point of the book was to show how wicked they were, and how YHWH destroyed them all. So maybe there's a deeper point to the myths you're talking about?
Plus, in the realm of dead religions, how sure are we about how they were practiced?
That's a fair point- a lot of other stuff happens in those eras, but as far as I can tell, Tuan MacCarrell only gives an overview to the monks.
With Irish mythology it can get pretty tricky to figure out what is going on. The pre-Christian Irish didn't write down the mythology. It was written down as a point of interest by local monks, who occasionally interject their own obvious bits from Genesis into the narrative (for example, the first wave of invaders are said to be outlaws who somehow escaped the Flood without Noah or God noticing. Goidel, whose name is the root for Gael, is said to have constructed what would eventually evolve into Old Irish out of all the "good parts" of the 72 languages that emerged after the Tower of Babel incident). The Tuatha De are variously described as gods, false gods, wizards, some spags who came from the North or people who descended from the skies in a cloud of smoke.

But, comparing their names to names found on the continent, it's apparent that they were at one point considered gods in Ireland. As far as the practice goes, we don't know for sure. Druids didn't like to write things down. No one knows who they were or.... what they were doing.
I'm just taking that one blanket account, which I interpret as garbled history of immigration. I can look into it deeper, but the running theme in the details seems to be allegory for the cycles of nature and/or how history tends to repeat itself.