"All animals are equal, but some are more equal than others." Explain how this came to pass and what the irony of such a rule was?
This "commandment" is the corrupt pigs' distortion of the original principles of Animalism, formed after the initial overthrow of Mr. Jones. Because the pigs educated themselves, they became superior to the other animals and were able to use propaganda to subvert the original, central idea of the Revolution--that all animals are equal.
This distorted maxim is ironic in several ways. Most obviously, the words themselves are ironic; how can someone be "more equal" than another? Also, after recounting his dream to the animals early in the story, Old Major warns them against adopting any of the ways of man, and to never become like humans. Yet it is through education, the use of machinery, bartering with other (human) farmers, and selling their own farms' young that the pigs are ultimately able to gain control of the farm.
Some might also argue that it is ironic because it's true.