Update of sorts. I just got watching another series, so now I'm on the fourth episode of Tears to Tiara. So far, it seems that the anime is showing how the Celtic tribes were forced out of mainland Europe hundreds of years ago from the Roman Empire, and they sought refuge in other places in Europe, and a lot of the prominent tribes are in the British Isles (Ireland included). In the 4th episode, a general named Gaius is pushing the one Gael (Celtic) tribe northward to Erin Island (Ireland). So far a wise sage has joined the group for the clan with Arthur, Arawn (dubbed the Demon King), Riannon, and an archer named Morgan.
Another interesting fact; in the time around this era, a Celtic tribe was forced to migrate and made home in Israel. Turns out that tribe was the beginning of the Galatians in the Bible.
https://en.wikipedia.org/wiki/Galatians_(people)
https://en.wikipedia.org/wiki/Epistle_to_the_GalatiansI read that beforehand in my Celtic World history book I bought years ago, but started to read it last year and in 2014 (I haven't finished it yet, as I've been busy).
EDIT1: Got to see a lot more of the anime. It's really good!! It turns out that the "Divine Empire" (Roman Army) had a general under them named Octavia. She valued her soldiers' lives more and fought with honor. She had a dispute with her leading officer in the army and humiliated him in a Coliseum fight. She was then imprisoned by her own armor, but was rescued by the Celtic tribe. The episode I just watched, she joins sides with the Celtic tribe and is now fighting alongside them against the Roman Army. There are also elves in this story, but they do not have pointed ears like we are accustomed to with knowing what an elf is. They have normal ears, but they have a lifespan greater than a human's in this story. Plus there are different factions of them, such as seafaring, mining/blacksmith, forest-born, and so on. It's interesting. One of the elves is named Epona, which if you know your Celtic culture, is a Celtic Goddess of horses.