Libircclient is a simple C library for interfacing with IRC. It's fully threadsafe, and relies on only one external dependency: GLib. You can get the source at https://bitbucket.org/dwfreed/libircclient/. It is currently maintained by dwfreed who can usually be found in #botters on freenode.
libircclient was originally at http://libircclient.sourceforge.net/, but because it hasn't been maintained in a few years, dwfreed decided to fork it to fix a few bugs (mainly no outgoing queue and a limited input buffer) as well as extending it a little bit.
seabird is one of the main current examples not written by the author. seabird is located at http://bitbucket.org/belak/seabird and is maintained by belak. seabird is, essentially, a troll.