Orwell uses irony to mock the vices of humans because the animals overthrow the humans and then the pigs bring back all of the offenses that the humans stood for.
Old Major describes the abuses of humankind in his speech at the beginning of the book. He explains that man is the only animal that consumes without producing. The animals would be better off without humans.
When Jones and his men forget to feed the animals, they rebel. They break open the shed and everyone eats. When the men try to stop them they run them off the farm. The men try to return, but the animals keep them out. Before you know it they are creating their own perfect world: Animal Farm. It is based on the principles of Animalism, an animal-based philosophy of communal ownership.
After much thought Snowball declared that the Seven Commandments could in effect be reduced to a single maxim, namely: `Four legs good, two legs bad.' (Ch 3)
The irony is that the beautiful animals-helping-animals dream does not last. Before long, the pigs begin slowly taking over. They begin by making themselves in charge, and then taking goodies for themselves such as the milk and apples.
The pigs Napoleon and Snowball begin fighting for power, and soon the animals are divided into two camps. Napoleon kicks out Snowball, and begins consolidating his power seriously. All of the abuses the humans were vilified for start to come back, in the form of pig tyranny.
Soon Napoleon is using the dogs as a security force, and executing animals for no reason. He sends Boxer, his most loyal worker, to the glue factory as soon as he gets injured. As it turns out, the animals are more cruel to each other than the humans were to the animals.
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which. (ch 10)
With this satire of commuism, Orwell points out that in a revolution the new boss is often the same or worse than the old boss. The animals had dreams of a utopia, but it turned into a nightmare. Ironically, things turned out worse for the animals when they were in charge of themselves than when the humans were in charge. At the end, you could not tell the animals from the humans.