Just to test, try temporarily disabling your firewall and antivirus/antispyware/antitrojan software and run mIRC. See if the CPU usage is still high. Once you know, you can enable those again. As long as you're not using the web and don't type weird stuff in mIRC that people may tell you to write, you don't need to worry about disabling those temporarily.
Also, when does the CPU usage go up? Is it when mIRC's running, but not connected? Is it when it's connected to multiple channels, or just one? Is it while sending or receiving files? More specific information would help. I run 2 copies of mIRC. My bot uses 0% cpu at all times, and Invision uses 3-5% due to a lot of hash tables (something I still need to try and cut down at some point).
