IronPython 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.