When comparing Trillian vs IRSSI, the Slant community recommends IRSSI for most people. In the question“What are the best IRC clients for Windows?” IRSSI is ranked 8th while Trillian is ranked 22nd. The most important reason people chose IRSSI is:
Screen multiplexing and remote connection options with screen or tmux.
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Pros
Pro In-house server support
You can set up a dedicated server to hold your communications. Great for businesses that want to build their own infrastructure.
Pro Multiple networks in one place
Among them Yahoo!, Facebook, Skype, Google, AIM, ICQ, XMPP, MSN. You can have multiple accounts associated with each service.
Pro Group chat
Trillian to Trillian group chat is available.
Pro Terminal-based
Screen multiplexing and remote connection options with screen or tmux.
Pro Themes
Allows the modification of IRSSI's look and feel.
Pro Free and open-source
Pro Quick and easy setup
3 or 4 commands and you're ready to chat with all your old friends.
Pro Perl scripting
Allows the user to create and use perl scripts to perform actions.
Pro Command line outputs can be sent to a channel or message
/exec -o figlet "Slant.co"
Pro Easy logging
Pro Persistent connection
Pro Has a plugin for encrypted communications
Cons
Con Supports all dead services.
Con No GUI
Actually a plus for some, lower memory overhead.
Con Gives little direction upfront
Actually, Irssi.org has it's own GREAT documentation for new irssi users. Inline /help options are pretty well documented.