I'm not well aware of how PPC works and how mIRC's installer works.
But you may try the following:
- install mIRC on a personnal computer
- transfer the files from that computer to your pocket PC

You may need to edit some files to set the correct paths to files.
Also, mIRC uses Windows' registry to store some informations (like your registration key), so you may encounter problems like being unable to register your copy even with a valid key, or mIRC may not run at all!

Best of luck anyway!