IronRuby maintains compatible versions with Ruby 1.8.6 and Ruby 1.9; IronRuby 1.0 and IronRuby 1.1.3, respectively. If you're not sure which version to use, use IronRuby 1.1.3, but only if you do not need Ruby 1.8.6 compatibility.
IronRuby is a cross-platform and cross-browser programming language, so prerequisites will vary based on usage.
Here are the recommended runtimes are for each platform:
Windows desktop & server apps
Windows & Mac OS browser apps
On Windows & Mac OS for browser usage, Silverlight is the recommended runtime.
Mac OS & Linux desktop & server apps
Linux browser apps
IronPython also runs on Moonlight, a open source implementation of Microsoft Silverlight for Unix systems.