Colloquy is a Mac IRC client that tries to keep the look and feel of a standard Mac app.
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 Has pretty themes like iOS "bubbles"
Pro Works on macOS Mojave
Although not supported anymore, it's been confirmed to work on Mojave.
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.
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.