If you are using the latest version of mIRC, the SSL options dialog comes preset with defaults that normally do not need to be changed.
The "Server certificates" option should be left at "Reject invalid certificates", since if a server is using an invalid certificate, there is no guarantee that the SSL connection is secure anyway. A "Private certificate" is usually not needed. The "Certificate chain" should be left empty. The "Trusted authorities" is usually set to the cacert.pem file that comes with mIRC, but it can be left empty in which case mIRC will use the Windows certificate store. The "Allowed ciphers" should be left with the default settings - click the "Default" button.
Depending on the IRC network/server, the SSL port can be +6697, although it can be other port numbers. You would normally need to find out by connecting to 6667 first to read the server's MOTD (the message of the day that appears when you connect) which will provide information on their supported ports for secure connections.