1) Make sure you have installed the latest stable or trunk version of Mono available for Mac OS X
2) Download the latest IronRuby source code from Github here
3) Unpack the tarball(tar.gz) or zip archive(.zip) file downloaded, then open a terminal in the source directory just extracted (i.e. ironruby-ironruby-b871551/)
4) Try to compile using xbuild /p:TreatWarningsAsErrors=false Solutions/Ruby.sln
(I'm using Mono 2.6.7 and when I try to compile the build fails because there is no /usr/bin/dmcs command in this version, so you need to compile Mono from Github code or from this Trunk source see this README file and also READ THIS )
5) If building from the main IronRuby git repository doesn't work you have two alternatives, one is described here, which requires you to use a different branch, and the second alternative is described here
I successfully compiled IronRuby using the second alternative using Mono 2.6.7 on my openSUSE 64-bit KDE desktop box.
Additional resources :
Building IronRuby with Mono on OSX
Getting Started with IronRuby by the author of IronRuby Unleashed
IronRuby, Mono and My Mac
Alternatives:
MacRuby
JRuby (Getting Started with JRuby)