Ironhax is an exploit in the USA/EUR eShop game "IronFall: Invasion" that allows people to load 3DS homebrew on system versions 9.5-10.0 and above. "IronFall: Invasion" was removed for initial download from Nintendo eShop as of August 11th, 2015. On October 13, 2015, the main app title was restored on the eShop with an update included with that title itself: since that version fixed the vuln used by this exploit, a version older than this one is required if you really want to use this exploit. Unlike other exploits this is a secondary exploit, meaning in order to install it the use of another exploit with the Homebrew Launcher is required. The Ironhax installer requires internet, afterwards the installed exploit will work without internet.
See here for *all* setup instructions: 3DS homebrew official site.
Sometimes the 3DS might get stuck at a blue screen or a white screen (bottom touch screen) and if this happens force shut your 3DS down by holding the power button until the blue light turns off and then try the whole exploit again.
Also keep in mind that Ironhax is a Userland exploit, meaning that it has only ARM11-userland access and doesn't have ARM11-kernel. This means we can't modify the system but that doesn't mean we can't do some awesome homebrew! Regionfree, emulators, custom homemenu themes, playcoin hacks, etc. are all available!