When comparing Colloquy vs IRSSI, the Slant community recommends Colloquy for most people. In the question“What are the best IRC clients for Mac?” Colloquy is ranked 3rd while IRSSI is ranked 5th.
Ranked in these QuestionsQuestion Ranking
Pro Free and Open Source
Pro Nice Native Design
Colloquy was designed with the Mac OSX interface conventions in mind, so it's got a native look and feel.
Pro Works on macOS Mojave
Although not supported anymore, it's been confirmed to work on Mojave.
Pro Has pretty themes like iOS "bubbles"
Pro Plenty of advanced features
Colloquy has many of the features an advanced user would need, such as file transfer support, and even support for mIRC colors. It supports scripting via AppleScript.
Pro Supports SILC and ICB
Pro Mobile version
Colloquy also has an iPhone version for IRC on the go with many of the same features.
Screen multiplexing and remote connection options with screen or tmux.
Pro Free and open-source
Allows the modification of IRSSI's look and feel.
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 Quick and easy setup
3 or 4 commands and you're ready to chat with all your old friends.
Pro Easy logging
Pro Persistent connection
Pro Has a plugin for encrypted communications
Con No longer supported
The app will not work on the latest releases of OSX as it has not been updated since 2014.
Con No way to search chat logs within Colloquy
Chat logs are not viewable within the app. You must go to the directory and open the file with a text editor.
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.