How do I set up an IRC server on OS X for my local network?
Since you have Homebrew installed, ngircd is very simple to install and configure.

Install ngIRCd

$ brew install ngircd


Add /usr/local/sbin to your PATH

$ vi ~/.bash_profile


edit the config file (very easy to follow, even for a beginner, as the defaults are just fine for everything.) Note the path may be different for you to homebrew / ngircd version folders.

$ cd /usr/local/mxcl-homebrew-697d2ae/Cellar/ngircd/17.1/etc/
$ vi ngircd.conf


Here's a sample config file, should you need it.

Verify your config, if you like:

$ ngircd --configtest


Then simply start the server:

$ ngircd