ForgeIRC

ForgeIRC is a mod by ShadwDrgn designed around interaction between Minecraft's in-game chat to IRC channels. This is for people who would like to chat in both chats, but would not/can not have both Minecraft and an IRC client open at the same time. It can also be used to talk to people who are not on the server you are on, or are not currently playing. ForgeIRC has different uses depending on if it is installed on the server/client.

Server Usage
If ForgeIRC is installed on your server, then it will relay all chat on the server to your selected channel, and all chat on that channel back to the server. It will not relay its own relay messages, as this would cause a loop. In the config file, you may specify which IRC server and channel the mod will join and what name it will join it with.

Client Usage
If ForgeIRC is installed on your client, then it will relay only your own messages to IRC, and relay messages from IRC back just to you. Messages from IRC will be displayed with the [i] prefix to differentiate them from other Minecraft messages. You may also press a "mode switch" key while typing in chat to determine whether your current message will go to IRC, Minecraft, or both. By default this key is F3. Just to clarify: this key only switches chat modes if you press it while typing a chat message in-game.

Like in the server version, you can select the server, channel, and IRC username for yourself in the configs, but if ForceIRC is installed on the client you can also press the ForgeIRC menu key to change the channel and username in-game. You must still select the server ahead of time in the configs, though. This menu key is M by default, but can be changed in the Minecraft options menu, as can the mode switch key.