Colloquy is a great OS X IRC client, which now functions with your iPhone as well!

How to Install
<ol>
<li>Make sure you have the <a href="http://colloquy.info/downloads.html">latest version</a> of Colloquy.</li>
<li><a href="http://colloquy.info/downloads/colloquy-web-interface.zip">Download</a> the beta Web Interface plugin.</li>
<li>Install the plugin in: <em>~/Library/Application Support/Colloquy/PlugIns</em></li>
<li>Create a new password that will be used to login from iPhone. Type the following command in Terminal.app with your password at the end:
<tt>defaults write info.colloquy WebInterfacePassword <strong>&lt;&lt;yourpasswordhere&gt;&gt;</strong></tt></li>
<li>Open or relaunch Colloquy.</li>
<li>Visit http://yourip:6667 on your iPhone.</li>
<li>Enter the password you created earlier.</li>
</ol>
<a href="http://colloquy.info/iphone/" title="Colloquy IRC for iPhone" target="_blank"><img src="http://www.modmyi.com/images/apps/colloquy.png" border="0" height="231" width="199" /></a>

Visit their page at <a href="http://colloquy.info/iphone/" title="Colloquy IRC for iPhone" target="_blank">http://colloquy.info/iphone/</a> for more setup and installation instructions.