When comparing IRSSI vs irc.c, the Slant community recommends IRSSI for most people. In the question“What are the best IRC clients for Linux?” IRSSI is ranked 4th while irc.c is ranked 16th.
Question Ranking
Pros
Pro Terminal-based
Pro Perl Scripting
Allows the user to create and use perl scripts to perform actions.
Pro Persistent Connection
Pro Command line outputs can be sent to a channel or message
/exec -o figlet "Slant.co"
Pro Themes
Allows the modification of IRSSI's look and feel.
Pro Free and open-source
Pro Has a plugin for encrypted communications
Pro Single server
Each instance of the client can only connect to one server and then you can join multiple channels with the join command.
Example: "irc -s irc.freenode.net", then "j #lua" and "j #chicken"
Pro Open-source
The source code (a single C file) is under public domain.
Pro Infinite scrollback
You can move up and down the history with the "Page Up" and "Page Down" keys.
Pro Automatic reconnection
It automatically connects you back if you happen to get disconnected.
Cons
Con Gives little direction upfront
Actually, Irssi.org has it's own GREAT documentation.
Con Cryptic configuration files
Con Can't do vertical splits
Untrue, there are perl scripts available to do split screens with
Con No GUI
Actually a plus, lower memory overhead
Con Documentation
Its website and README do not give enough information about shortcuts and commands. You have to look those things up in the source file.