If you wanted to call someone "Katie of the Fairies" in Irish, would that be "Caitlin ó Sidhe" or would "de" be more appropriate? And does Sidhe need lenition in either case?
Go raibh maith agat.

I don't think you need either ó or de. You could just say "Caitlín na Sí." It puts Sí into the Genitive case (of the) and doesn't need lenition.

Go raibh mile maith agat! I'm still having trouble with cases and tenses and don't always see what should be obvious. Back to the studies...