That word always used to be spelled "crack", until at some point in the last few years somebody started spelling it as "craic",
Well for heaven's sake--I wish they hadn't done that, for a very odd reason. I knew someone who bandied the word craic around with all kinds of fake knowledge: showing off, I felt. Had the word been crack, it wouldn't have gotten on my nerves so badly, because crack's not a foreign word to US'ns.