I've used the IRC for at least 10 years, if not more, and I've utilized a range of clients starting with the stalwart mIRC for Windows. When I migrated my home system to a Mac, the IRC client of choice quickly became the open-source Colloquy. It was simple to use and easy to integrate scripting via AppleScript. I could easily join chat rooms across multiple servers
So, it should be no surprise that Mobile Colloquy [link opens iTunes] shares many of the same features as its desktop kin. We first saw Mobile Colloquy back in 2007 when it was made available via the Installer.app and it very much the same on the surface as its jailbroken predecessor. It's taken the promise shown back then and has turned into an excellent and stable release that's now available through the App Store.
One of the unique features is a built-in browser that will open links transmitted in a chat room or private message. The some of the pages opened through the link are not iPhone-optimized version, but the regular Web sites. I accessed the optimized versions of CNN and Google just fine through the built-in browser, but TUAW came up as the normal site.
The button on the bottom right of the browser will take you into Mobile Safari. There is no bookmark tool, you'll go into Safari to take care of that. The "Done" button closes out the built-in browser and returns you back to your chat. It's a very nice feature, but I wish you could bookmark links without having to open the same web page again in Safari - something that could be extremely tedious over the EDGE network. Another drawback is when you do choose to go into Safari, you will be knocked off the servers you were logged into. Give the client a few seconds for the servers to reconnect, then tap the name of the chat room you were in. You will get a prompt asking you if you want to rejoin it.
Another very nice feature is completion pop-ups for basic IRC commands, including user names of other people in the chat room. You can also use specialized commands, such as /google query and /wikipedia query, which will bring up results in the built-in browser. All chat rooms and private messages are gathered under one screen. You can set specific words to be highlighted and turn on an optional vibration when the word is used, such as your nickname. Colloquy can be used either in portrait or landscape mode as well.
Mobile Colloquy is available in the App Store for $1.99, and it's well worth the money.