Well, it's not straightforward to say. If by Big Iron you mean a specific story or work, it could be a blend of real and fictional elements. But without more context, it's hard to determine for sure if it's completely real.
The Iron Monkey's real story is one of a Robin - Hood - like character. He operates in the shadows, stealing from the rich and corrupt officials and giving to the needy. His deeds are passed down through generations, becoming a symbol of resistance against oppression. He is highly skilled in martial arts, which he uses to outwit his enemies. His actions inspire hope among the common people who are suffering from the tyranny of the powerful.
The 'Iron Fist' is based on the Marvel Comics character. In the real story, Danny Rand, a rich kid, was orphaned after his parents died in a plane crash. He was then raised in the mystical city of K'un - Lun. There he trained in martial arts and became the Iron Fist, a powerful warrior with a glowing fist. He returned to New York to reclaim his family's company and fight crime using his martial arts skills and superhuman abilities.
No one knows for sure who the real man was. Some historians suspect it could be a political prisoner, perhaps someone related to the royal family in a way that made his existence an embarrassment. There are so many speculations. It could be someone who knew important secrets about the court. However, there is no conclusive evidence to firmly identify him as a particular person like Eustache Dauger or anyone else.