What about having a dialog in mIRC that allows you to manage multiple mIRC data folders? Here is how I think it could work:

- User runs mIRC installer
- Installer creates serial number <sn> for new installation of mIRC
- Program files are installed to \Program Files\mIRC_<sn> (or user chosen location)
- Data files are installed to \Application Data\mIRC_<sn>

- After installation, user can display a setup dialog to choose which data folder is used by each mIRC instance.

*I'm making this up on the fly, so it undoubtedly needs some work.