If you're looking for a feature-filled IRC server and have a fast server to boot, Unreal will be one of the most useful for you. With a vast number of features, and an equally large number of modules (addons), you can tweak this server to your heart's content. While there are a large number of IRC daemons (server applications) today, almost all of them are forks of other server applications. For example, UnrealIRCd was a fork of EliteIRCd. They then went on to add lots of features to the codebase to make Unreal what it is today. The new version of UnrealIRCd in development right now - 4.0, is a fork of another well-known server daemon/application, InspIRCd.
UnrealIRCd runs on Windows, Linux and BSD (Unix) operating systems. You will need to download the corresponding archive for your platform. Linux and Unix archives will be source files which will have to be compiled manually before installing. The Windows archive is a pre-compiled binary which is to be installed directly. Helping you configure UnrealIRCd is beyond the scope of this article since the configuration file starts at almost 800 lines and can go up to 1500 lines depending on your configuration, but if you're comfortable with large configuration files and networking/ip-addressing, you should be good to go. The official docs are very good and will help you with whatever problems/queries you have.
UnrealIRCd Website. (As of writing, this website is down. It should hopefully be up soon)