Saint Brigid of Kildare

Question: Would Brigid being revered as Saint Brigit by Irish Catholics have a major impact on her personality? Would she be more tame and reserved than the rest of the Tuatha?

That entirely depends on how you want to run syncretism in your games.

Brigid's a very classic example of religious syncretism; while not one of the most important of the Irish gods, she became extremely well-known thanks to her syncretization and eventual absorption into the character of the Catholic Saint Brigid, who has plenty of magical tales and ideas attributed to her thanks to the ever-wonderful embellishing nature of Irish Catholicism. She magically heals people, she predicts the benevolent future, she curses bad people with misfortune, and so on and so forth, generally having a lot of magical elements, like most saints, to back up her holiness and divine favor.

You certainly could decide that these elements might have influenced and shaped the character of the goddess Brigid; after all, Scion's setting includes Fatebonds, so if she were to have accrued enough of them when St. Brigid's legends were being born, they might have caused her to change with them. But if you do so, you have to be aware of the implications for all other religions and deities, because Brigid is far from the only one who suffers from strange syncretization.

If Brigid is going to be affected by the persona of St. Brigid, then will you also be changing Tlazolteotl to better resemble the Virgin Mary? Will you be reframing Baldur as Christ, Baal as Beelzebub, Vishnu as Buddha? Will you be considering all later syncretized forms of Egyptian gods to be closer to how the Greeks thought of them than the Egyptians? What about the Hindu and Shinto pantheons - are they all only what later Buddhism may have made of them? Where are you planning to draw the line when it comes to what syncretization by humanity does and does not do to a god?

The rife cross-culturalization of many ancient religions means that we generally don't give deities powers and stories based on their associations as saints, demons or members of other pantheons, because doing so usually means losing much of their original, unique cultural flavor in favor of dragging in someone else's religious view of them. We prefer the gods in as close a form to their heyday among their own people as we can get them (which varies depending on source material, but we take what we can get!), because part of the fun of Scion is the vast difference between different mythologies and the cultures they come from. Then, too, there's the fact that Christianity and other syncretic religions like it are comparatively young; in Scion's setting, most if not all the gods should have stopped meddling in the World by the time they became popular, which means that they wouldn't be gaining Fatebonds and it doesn't really matter what humanity decides to associate them with because Fate's power isn't behind their syncretizations or comparisons. Just as we've talked before about treating monotheistic religions as largely human inventions, so would their associations and attempts to absorb other religions be mostly human happenings.

Now, I could see Brigid being affected by St. Brigid's stories, but there would need to be very good reasons for it. You'd need to know, as a Storyteller, why she was still running around in the World getting Fatebound when the pantheons had long ago withdrawn, why she did the things she did as St. Brigid, what it might have done to her personality and what the long-ranging effects of all this was, in more dimensions than just what she has associated for XP purposes and whether or not she has a lot of dots of Courage or Piety (also, note that having Piety makes it really, really hard to pretend you're part of a different religion). More importantly, you need a good reason to do all this work, especially if you aren't planning on doing it for every god in the game. If Brigid is directly involved in the plot due to PCs' origins or actions, then you have a reason to work on her - but the answers to the questions about need to have something to do with the plot and the PCs. If the PCs or the plot they're heavily involved in make it so that Brigid needs to be involved and needs to have something to do with Catholicism, then by all means, have at it - but if not, I would steer clear of it lest you open a can of neverending, ravenously syncretic worms.

Generally, we leave Catholicism, along with most other monotheistic and more modern religions, as human creations that probably don't affect the gods much, because it's simpler and stays closer to the themes and ideas we enjoy in Scion; the gods may use them when convenient (particularly the Loa are notorious for doing so), but they aren't defining factors of their personalities. You can certainly decide to make syncretism a major theme for a particular deity, but doing so is a large undertaking, and if you don't have a solid reason to do so, it more often than not may be a lot of window-dressing busy-work for the Storyteller that doesn't meaningfully connect to the game the Scions are playing in the first place.

More interesting to me than trying to stuff gods into their syncretic roles is exploring how they feel about them; some might find them amusing, others might be offended, and I imagine a lot of them feel it's more imperative than ever not to go getting Fatebound in the World if that's what people are going to turn them into when they do.

This is why they need Scions. They so do not want to get any of that on them.