Midas Mulligan is another one of our legendary, larger-than-life, heroic figures in the book, comparable to characters like Richard Halley and Galt himself. Like Galt, Midas is linked to a Greek myth, one appropriately dealing with wealth and greed. (See the chapter summary for Volume 1, Chapter 10.)
In his long career, he had ignored all the public attacks on him, except one. His first name had been Michael; when a newspaper columnist of the humanitarian clique nicknamed him Midas Mulligan and the tag stuck to him as an insult, Mulligan appeared in court and petitioned for a legal change of his first name to "Midas." The petition was granted. (18.104.22.168)
Now that's hardcore. Mulligan has a lot of similarities to another outrageous figure in the book: the bold, legendary Nat Taggart. In this way, he ties in to themes of the past, like Richard Halley and Hugh Akston do. He seems to belong to an earlier age of industrial giants, and his disappearance also appropriately has a touch of the epic to it:
There was – Dagny had thought uneasily for years – a quality of the impossible to Mulligan's disappearance; it was as if New York skyscraper had vanished one night. (22.214.171.124)
Like a skyscraper, Mulligan continues to be a "towering" sort of figure in Atlantis. (He owns the valley, for one thing.) He can be downright gruff and seems to have left his internal censor back in the real world. It's questionable as to whether he says these blunt things without thinking or deliberately.
In the scene right before Dagny makes her decision to leave Atlantis, Mulligan goes on a rant about how bad things are about to get in the real world. Makes us question whether he's trying to dissuade Dagny from going back, in his own gruff way, or whether he doesn't realize the effect this speech might have on Dagny.
We get the sense that Mulligan was always like this, though, which is interesting, given how Atlantis transforms a lot of the strikers, like Ellis Wyatt. The Mulligan of Atlantis seems to be just as bold, blunt, and brash as the Mulligan of Chicago.