You can enable/disable the irc:// compatibility when you install mIRC I believe, I don't recall out of the top of my head wether there's an option to do so once the program has been installed (short of running the installation program again which of course is trivial..), but click around in the options and check.
And of course, if there's no option for it, and you don't wish to re-run the installation program, there's bound to be a registry entry you could modify/add to get this functionality anyway.
