A programer know Windows is a platform appart. mIRC must be using a lot of API calls which are not portable to Linux and other Unixes (x-window or other), nor Mac.
Each GUI is distinct in working and coding from other GUI platforms. No compatibility. Unless your program was thought of to be portable from the beginning.
Releasing mIRC as open source would be nice, but wouldn't allow it to work outside Windows.
Why do you insist to user mIRC on other OSes? There are already IRC clients on all platforms I know of. If you are not satisfied from them, modify them (some are GPL). Or start a new one from scratch with portability in mind.
