The story of Animal Farm revolves around the bitter irony that revolutions begun with the best possible intentions can soon degenerate to the point that the governments they create resemble the ones they were trying to destroy. This irony is best conveyed by the last paragraph in the book, when the animals are looking in a window at a table, where the pigs are playing cards with Pilkington and the humans:
Twelve voices were shouting in anger, and they were all alike. No question, now, what had happened to the faces of the pigs. 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.
The society that the pigs led the way in trying to achieve has now, ironically, reached the point where it is indistinguishable from the society that they sought to destroy in the beginning. The pigs, "more equal than others," are now walking around on two legs, carrying whips, exploiting the labor of the other animals, and carousing with the humans.
The most ironic scene is probably the card game at the end, in which Napoleon and Mr. Pilkington make hypocritical, congratulatory speeches to each other. This is ironic because Napoleon and Pilkington are really two sides of the same coin: each has never trusted the other, both are conniving, and both want to have the most power, as shown when they both fight over the ace cards which they have drawn.