What is ironic about Mildred saying "I understand that one" in Fahrenheit 451?
I guess you can say that this is ironic, although I think you can also argue that it isn't.
If it's ironic, here's how. What she says this about is when there is the quote in the book saying "that favorite subject, myself." What is ironic about this is that, you can argue, Millie does not even care about herself. She is so unaware of herself that she doesn't even realize that she almost killed herself. All she wants to do is watch the parlour walls and she has no self-awareness.
So it's ironic for her to say that she understands being interested in herself.