Bullfrog, you beat me to it. But the "-ix" spelling seems mainly reserved for the word as term of abuse to describe a person.

When rubbish or nonsense is referred to, the "-ocks" spelling seems to be preferred. Socio-ethnoetymologists please discuss.

Good thread, Philippa, and lots of solid research to sustain the judg(e)ment that, on balance, "crack" is the more valid spelling. Time will tell whether "craic" will differentiate itself sufficiently to make it into the OED, which is more prescriptive than Merrriam-Webster (cf. historically correct entries on "concentration camp"). But I do find "craic" a bit irritating, mainly because its principal perpetrators are pub owners trying to snare innocent tourists and convince them that the Irish did indeed invent fun (we didn't invent it: we're just one of the last Western European nations to lose our sense of it, but a bit of economic prosperity and associated McCreevy-style arrogance should soon fix that).