In chapter one, Old Major (the Karl Marx figure) tells the animals that he has a dream for the future of Manor Farm. His dream is that one day the animals would be free from the oppression of man. He saw a future where animals would no longer be whipped or led around by the nose by a dictatorial figure who didn't have their best in mind (like Farmer Jones (the Czar Nicholas figure)). He hoped one day the animals would rule themselves and that "All animals would be equal." (Communism in it's ideal state).
Considering the fact that this was what inspired the animals to revolt and take over their own farm, it is extremely ironic that they allowed the pigs (more specifically Napoleon) to become exactly what they rebelled against. In the end, Napoleon became drunk with power (one of the themes of the novel is that power corrupts) and put himself and the other pigs above the other animals. He saved the best for himself, and allowed the lower animals to work harder and have less.
In fact, in the end, the humans praised Napoleon for treating his animals worse than they treated theirs. So the irony is...the animals created a society that was the same as the one they were trying to get away from.