mIRC is not illegal in any way, shape, or form. mIRC can be used to do illegal things, but so can Internet Explorer. If you're talking about sharing copyrighted materials then yes, that is illegal in almost all countries, but you'd need to be living under a rock not to know that by now.
You should note that it's incredibly bad form to question the legality of mIRC (or any other business/service) without providing any sources whatsoever. See also:
FUD
