Don't forget Mt. Norquay in Banff . . . and Nakiska, site of the '88 Olympics . . . Although, Nakiska is rumored to not be the greatest hill for skiing . .. Sunshine is definately the best (skiing-wise), though.
The Big Three hire mostly people who apply from Canada. They have a weekend hiring fare. You show up and get a job, basically. Also, I think they offer staff accomodations.
You can also try Panorama that way. They hired me as a housekeeper (though they probably hire ski lift operators, etc. that way as well) and they have staff accomodations that are pretty nice plus free skiing during your downtime . .. and it took about a couple weeks to get everything in order. It's easy to get a job when you land. You don't need to plan ahead that much. Many people fly by the seat of their pants, finding work as they go.
Whistler, of course, is the best and would be awesone during the Olympics. It would be advisable to get in early, and it is certain to fill up quickly. There are tons of ski hills in the BC Rockies, though, like Silver Star, etc., so if it's too late for Whistler, it'll be probably easier to find a placement near Whistler.
My advice would be to first try the internet, googling ski hills in the Canadian Rockies, and even applying at a few online, if you are interested, or by emailing/faxing your resume to the apropriate people. If you go the more spontaneous route, bring your printed resume with you and be prepared for a bit of downtime for logistics to come together, maybe about a couple weeks. Stay somewhere with your own phoneline (cheap motel) or bring a cell/mobile, and make sure you have some sort of fixed address or you will go nowhere. It is possible to do it this way, but you might find it easier to fax and phone from your home in Ireland (I hope you have a long distance plan). However, many people find it easier to do everything once they land. It is really up to you.
Good luck! 
Steph