I’m coming across this phrase (used as a greeting) in The Once and Future King quite a bit. I’m charmed by it. Does it appear in any of the medieval romances, or did T.H. White make it up?
Sounds like Irish, it is the response to “hello” (dia dhuit = god be with you), the response is “Dia 's Muire dhuit” (God and mary be with you). I think Patrick comes next (God and mary and patrick be with you) but I could be wrong, after that all bets are off.
Should be “Mary and God.” Always mention the lady’s name first.
No, that would have been in Mists of Avalon. 
This sounds right; I’m recalling at least one exchange where Patrick got brought up too. Forgot to mention, it’s those wild&woolley Orkneys who use the phrase most in TOAFK. Not Irish, but still. Not Camelot. Countrified.
Eve: My mama brought me up right, and I let her down! 