A new IRC client debuted on the iPhone today and is available for download from Installer.app. Like many other iPhone projects, Mobile Colloquy is still in its early days, taking baby alpha release steps, but it shows excellent promise. It allows you to create server profiles, enter your user information and, of course, connect to those servers and chat.
Although Mobile Colloquy uses the same core as Mac OS X Colloquy, the two projects are not affiliated. For any issues with Mobile Colloquy, the developers request that you please file reports at the Mobile Colloquy Google Code site and not at colloquy.info.