I'd advise against IRC's... they are just a huge pain in the ass for everyone involved. If your bank lets you do currency exchances, thats an option so you could just sent the celeb some of their own currency to cover postage. However if you just send an SAE, that'll work too. But YES it will affect your success rate. I very rarely send SASE's to international celebs but I still have a pretty good return rate. I'd probably have a slightly better return rate if I sent SASE's but oh well lol

It's best to include a SAE with IRC's

You mean a SAE right. That's fine. Some stars will pay postage and some won't. You could also exchange your country's money for US money and put about $2 in your SAE. The place you send can use the money to pay for postage. It's polite.