how is the reality of their situation ironic when compared to their first visions of animalism?

According to Old Major, Rebellion is the path to freedom. Overthrowing the human race would make animals “rich and free” almost instantly. Old Major begs the other animals to devote the rest of their lives to the cause of Rebellion and to reject the idea that they have co-dependence with Man.  His dream was about the state of happiness that will exist once Man is eliminated. The irony in their dream can be found in the fact that they traded in one tyrant (Mr. Jones) for another (Napoleon).