Lore

Bith, Birren, Settlers, Ban people, blah blah blah who the hell are all these people the lore constantly mentions?

They're from Irish mythology, as usual.

http://en.wikipedia.org/wiki/Cessair

I figured they were taken from the myth, but remember Mabinogi has different interpretations of the characters from the myth (Glas Ghaibhleann isn't a cow that gives endless milk, for example) so there might be key differences.