Q. An agent in the States asked me for an IRC with my script. Whazzat?
It's an International Reply Coupon. You buy one at, say, the Canada Post, and when the agent wants to send your script back, he can slap it on the envelope. Because, obviously, Canadian stamps don't work in the US.
Of course, you're probably better off telling them they can recycle the script if they don't like it. Probably cheaper than shipping it back, even assuming it's not thrashed. And by then you ought to have rewritten it, anyway.