I hope you’re keeping well.
Fionn and the Salmon is found in the Macgnímartha Finn (Boyhood Deeds of Finn) and is part of the Fiannaíocht (Fenian/Ossianic) Cycle, whereas the Book of Invasions or Lebor Gabála Érenn is part of the so-called Mythological Cycle. However, there is such an enormous corpus of Irish literature that it is hard to remember which cycle and where everything comes from! And to think how much may have been lost too!
As for the Dagda, it is indeed odd that the “Good God” would be portrayed in such unflattering ways in the mythology, isn’t it? Was this perhaps a way of saying that the truth isn’t always pretty or that good medicine isn’t always nice to taste? Or was it perhaps that the Christian monks who were writing down these tales were keen on portraying the Dagda as a primitive, barbarous divinity, i.e. they weren’t going to cast him in such a serious or positive light given their own beliefs. Although the manuscripts we have usually date to the 11th-12th centuries, they may have been written down earlier, when pagan Norsemen were attacking Irish monasteries, so the monks, who would not have held anything pagan in such a good light anyway, may have been even more scathing. However, this is yet again the realm of conjecture. A much more flattering portrayal of the Dagda can be found in the Cóir Anmann (The Fitness of Names).
Just how divine were these figures? It’s hard to tell, but I think that we have to try and imagine how they might have been conceived by the ancient, pre-Christian Irish, rather than with our own modern view. What is apparent is that these superhuman figures are certainly larger than life and yet they are very human in their “failings” and have their own weaknesses and mortality too – Nuada loses his hand in battle, Cermait is killed by Lugh and so on.
As for Balor, I’ve read one interpretation that sees him as being the negative aspect of the sun, interesting that he is killed by his grandson Lugh, possibly the “Shining/Bright One”, who might represent the positive aspects of the sun. That etymology of Lugh is disputed, however, it is still interesting in that Lugh is the grandson of a Fomóire. It makes you wonder if we haven’t a deeper reading here, with ideas of cycles of birth and death going on – very fitting for solar mythology.
Anyway, just some of my thoughts and input. Make of it what you will.