The irony of gender
Although gender is signaling physically that there are differences between men and women (there literally are biological and medical differences between the two), that doesn't mean anything more than it means. It is ironic that people have so commonly and ubiquitously assumed that just because women are smaller, they are also incompetent. Are we really sexist just because women have been historically smaller? Mill says yes, this is obvious and it is wrong.
The irony of disenfranchisement
Ironically, the injustice of society involves disenfranchisement from power, so it takes sacrificial people in power who will do the unpopular thing of sacrificing their rapport in a broken system to re-enfranchise those who have been disallowed to represent their point of view. In this case, Mill notices that women never have been perfectly enfranchised in his society, not since the dawn of man.
The absolute obviousness of the problem
The problem is obviously that men chronically disallow women to be successful and then they hide behind the results as if the failure of a woman to become powerful (against the assumptions of the men in her life) is evidence that they were right to prevent her from growing. Obviously, this is a problem, and this is the crux of Subjection.
The ironic obviousness of the solution
Obviously, if the problem is that women are not encouraged to grow, thrive, and try and fail, they cannot become powerful, since men have to be encouraged to grow and thrive, and since they must try and fail, learning from their mistakes. Ironically, these problems are emotionally rooted in stubborn attitudes—it isn't that they are necessarily complicated problems. Mill says the answer is obviously to support all people equally, both men and women.
Feminism as a painfully ironic endeavor
Therefore, the most crucial irony of the book is that it is painfully ironic to have to have these conversations in the first place. Whether a person identifies as a "feminist" per say, or not, it is ironic to argue that women are less than men while literally manufacturing a system that would prove that assumption correct. As Mill notes, if no one imposes rules at all, natural law will govern women. Men need not involve themselves whatsoever. If a woman "cannot" do something, then it will be obvious; to prevent her from trying is painfully counterintuitive.
