A good example of irony is the juxtaposition of living conditions on the farm both before and after the Revolution. While Old Major's philosophy is based on a general sense of exploitation by the humans, the animals are generally well-kept and only occasionally mistreated by Jones, who is regarded as a below-average farmer. However, in the spirit of rebellion, they see this treatment as inhumane, resulting in the expulsion of Jones. After the Revolution, and after Napoleon has fulfilled his destiny as the Dictator of the Animal Farm, the animals are actually worse-off than before, since Napoleon is deliberately squeezing them for work without care for their well-being.
...their life, so far as they knew, was as it had always been. They were generally hungry, they slept on straw, they drank from the pool, they laboured in the fields; in winter they were troubled by the cold, and in summer by the flies. Sometimes the older ones among them racked their dim memories and tried to determine whether in the early days of the Rebellion, when Jones's expulsion was still recent, things had been better or worse than now. They could not remember.
(Orwell, Animal Farm, msxnet.org)
The irony is that although the animals believed that their new philosophy would result in a Utopia, it resulted in a Dictatorship. They would have been better-off accepting Jones and the farm status quo, as it was simply a working farm instead of a work camp. Their noble ideals led directly, and inevitably, to Napoleon's rise and rulership, just as Orwell perceived in reality.