Didn't know that. The 'Mi' is the same as Italian ie as in 'Mi scusi' but reverse translated in English to mean 'Excuse me'. As for Diar...Celtic (Gaelic/Cornish Breton) is not a branch of the language tree that i have studied.
'Jesus Christ' dependent on the context would be deemed as profanity, ie using the Lords name in vain. I have known people to be offended by it's usage (not always openly). I'm not really sure how Diar Mi fits into that same definition but as i said earlier i've never studied that branch of language. Mi = Me, so no profanity there. That leaves Diar - I know nothing about the Celtic term/meaning of this word but if you are able to give me an example then i'd be interested.
I could find no translation for 'Diar' in English. But in Italian it translates to perbacco which indeed means what you have translated. However, slight oversight by Matth as my curiosity got the better of me... It appears in the Urban dictionary 'diar' means; 'I'm diar. I'm gay.' therefore used in the context that Matth constructed the wording 'diar mi' - sorry Matth but you just came out, we are proud of you
