Ironhax is an exploit in the 3DS's Ironfall: Invasion game from Nintendo Eshop (was never available in Japan Eshop) that allows people to load 3DS homebrews. This works on versions 9.5-9.9. Ironfall: Invasion is not available for download anymore as it was pulled from Nintendo Eshop as of August 11th, 2015, so you needed to have it downloaded already to make this work. Unlike other expoits this is a secondary exploit, meaning it requires another exploit with the 3ds homebrew launcher for this to be installed. Ironhax requires internet only the first time running it and will work without internet after the first time.
Download Ironhax: [1]
Setup instructions available here: [2] (scroll down to the IronFall section down the page).
Like other exploits, Ironhax will launch boot.3dsx from the root of the SD card so make sure you have The Homebrew Launcher which is included with the 3DS homebrew starter kit which is available here: [3]
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 access and doesn't have 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!