Phillipa - I had never heard the word 'craic' used to describe lively conversation until I was in Ireland. It was used as in, "There was some mighty craic in that pub last night." Some pubs were well known for the craic. As a visitor, thats what I wanted (in addition to the music and a pint).

Crack has many other English meanings but I have never heard it used to describe converstion.

In fact, I have come to think of it as particular to the conversational ability of the Irish which is highly developed when compared to the English, Canadian or American.

Go for craic! Its unique.