yes it's illegal. you are allowed to offer the script as a download, but you may not include mIRC in the package.
To make it even more clear, this is what the license agreement say:
mIRC may only be distributed as the original distribution install file as distributed by mIRC Co. Ltd. The mIRC distribution install file may not be distributed as a part of any package, and may not have files added to it or removed from it, and none of its contents may be modified, decompiled, or reverse engineered.This means you can not distribute the mirc.exe file as a part of any script, it would also be a waste of bandwidth to put a huge .exe file in your .zip file.