FiSHLiM is a plugin for FiSH IRC encryption. You set a password for a conversation/channel, and then all your messages get encrypted. Only those who know the password will be able to read your messages. Usage is simple: first you set the password:
Then you let your fellow chatters know this password. Once they also set the password on their sides, they'll receive messages decrypted and send them encrypted, too.
The source code is available in the repo.