Foxit should be putting an entry into the Registry when installed, so nothing special should be necessary for non-portable versions of Iron; a similar thing goes for Flash, but if you want the special Chrome builds of Flash and the Chrome PDF reader...
Download the Dev version of Google Chrome Portable here: http://portableapps.com/apps/internet/g ... e_portable
Install it to any folder you like: You'll be deleting it soon anyway.
Then in the folder containing "chrome.exe" you should see another folder with a version number in it; in that folder you will find "gcswf32.dll" (Flash) and "pdf.dll" (Chrome PDF), and you should copy those to the SRWare Iron program directory (the one containing "iron.exe").
I don't think something similar exists for Linux or OS X: Instead you would need to install Google Chrome and then copy the bundled plugins.