The I.R.O.N. is a group of hulking testosterone-addicts. They are incredibly strong, but friendly unless angered, even helping new recruits. I.R.O.N. is led by The Tester, who died in the DOTC canon.
I.R.O.N. has a partner group called G.O.L.D, who use estrogen instead of testosterone.
Death of the Computer
After the death of The Tester, I.R.O.N. split apart. Some lost the motivation to work out, while others work out even harder to honour her. Some members even formed a new group, called the S.T.E.E.L. The Horde also contains an I.R.O.N. member.
War to Take the Surface
Now free from The Bunker, I.R.O.N. creates their own country called Testerland up in the mountains. They stay neutral in the war, accepting refugees if they promise to get swole. One of the members also found some pre-event workout tapes, so now they all talk like Arnold Schwarzenegger.
Not much is known about the I.R.O.N. in the Deep Dark canon. However, two members are seen, one following The Rope, wanting to find the others, and one working for the Dark Watch.