Did you delete the folder where you installed mIRC as a portable application? This folder will contain all of your custom settings and files. These are not deleted by the uninstaller, as recommended by Microsoft guidelines for uninstallers, which state that all of a user's custom settings and files after using an application should be left in place.
If you have previously installed mIRC without using the "portable" option, you will find your mIRC settings in your Users AppData\Roaming\mIRC folder, which is the Microsoft recommended location for saving user's settings and files (when not running in portable mode). You may need to delete that mIRC folder as well, if it exists.