"Craic" is a relatively modern transliteration of the English word "crack", and leads to all kinds of misunderstandings when people try to make it sound more Irish by omitting the article (if you say "we had a great crack" you may still confuse, but you're less likely to shock). The matter was gone into in some depth in the discussion Pedantic Crack, also touching on inappropriate use of the neologistic "sesiun" and the fantasy origins of "didgeridoo", before wandering off into the usual complete obscurity.