It seems that you already know what to do - just download the binaries and run them. For people who do not know how to achieve this: download the latest Mono for OS X and download IronPython-2.6.1-Bin-Net20SP1.zip or other Net20 binaries (Net40 does not work with this answer) then unzip the archive and call Iron Python by typing "mono ipy.exe" or "mono ipy64.exe" in the directory or try running the script "mono ipy.exe script_name.py".
Adding IronPython to MonoDevelop is planned, but does not seem to have any target date; if you want this, you need to go to the MonoDevelop website and say it or it will not be a priority.
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