Basically mIRC uses a Windows API call to check if a filename is a DOS device; if so, it will not allow access to that file. That Windows API call is not actually meant to be used this way though, as it also returns device names that cannot be used like filenames in the way that CON, NUL etc can. As it happens, those "ip", "global", "shadow" names are such device names used by Windows. You can get the full list (which differs from system to system) with
this utility.
