I'll usually put in in there right before my 15 minute hop addition, then add the hops after the wort comes back to boil (it'll stop boiling for a few seconds when you add the chiller). It's part of my routine; chiller, Whirlflock, 15-minute addition.
Oh, so lack of routine is why I forget the Irish moss about 80% of the time then? I swear, I have the same baggie that I bought like two years ago. I think my beer clears just because it knows I have the irish moss in the brew area and I'm not afraid to use it.