Well, it's a username I have on a few accounts...
...so I guess I'll just tell you where it originated.
One day, I was reading an article about one of my teams, and a key player was injured in a previous match. There was still speculation about whether he would play the next game, even though it was a foregone conclusion that he wasn't going to. This sentiment was eloquently summed up by someone in the comments section of the article, who wrote:
"It doesn't taken Ironstein to work out that he's not playing."
I thought "Ironstein" was brilliant, so I've used it ever seen. Of course, in a lot of cases, Ironstein by itself is taken, so I use "theironstein".
I sincerely hope whoever takes the name uses it in an ironic sense like I do. Otherwise, it might be a little sad.