Doth the sidhe?
On a more serious note, in alpha testing in mabi way back when there actually was Sidhe in the game. But obviously it never made it into the beta/public builds. And for a long time I never really understood why, but it kind of makes sense. According to celtic myth, after the Milesians invade Ireland, they overthrow then dominant Tuatha de Dannan, and forced them to live underground. It was there, underground, that the Tuatha evolved into the creatures known as the Sidhe and created the realm of Avalon/Faerie. I'm not sure what the story of the Alpha was, but it kind of wouldn't make sense for Sidhe to be in mabi if there are still Tuatha there (as it still is in the final product), unless they pull a generation where the Tuatha begin to evolve into a higher form and antagonize the milletians (which to be honest they have been doing in saga 1).
On another note, Mabinogi 2 was going to take place in Avalon, so in all likelyhood that game would have taken place after the at first benevolent invaders known as the Milesians overthrow the Tuatha regimes.