irssi is a free, command line IRC client. That means there's no graphical elements, no tabs and mouse use is limited to copying and pasting. If you want these normally essential features, consider X-Chat or mIRC instead. But otherwise, this IRC client offers speed and flexibility that few others can top.
The Advantage Of Age
One thing you need to know before trying irssi, though, is this: you'll need some time to learn it. Rather than clicking on tabs to switch between IRC servers and channels, for example, you press the Alt key and a number that corresponds to the chat window you want. Only somewhat experienced command line users will find the program comfortable to use at first. On the other hand, the program offers many unique features, most of them optional: you can integrate multiple channels into one window, run a bot through the client itself, and even read and compose Twitter replies in your IRC client, if you so choose. Learning to do all of this requires a bit of effort on its own, of course, but there's a lot of documentation that should help you learn the basics.
A Fast Choice
The biggest advantage is that a keyboard-only, command line IRC client is faster, both in its interface and how you use it, than other IRC clients. If that sounds worth the learning curve to you, then definitely try irssi.