According to Types of Irony, one example of irony in "Animal Farm" by George Orwell is the dramatic irony when the reader knows the money the pigs received from selling Boxer to the slaughterhouse is used to purchase more whiskey. Another example is the animals fighting for freedom from humans, only to solidify the power of the pigs, who act almost human by the end of the book.
SparkNotes mentions that irony is quite present in chapters III and beyond of "Animal Farm." The animals believe they know why they are fighting. However, during the Battle of Cowshed, they do not quite understand why they are fighting the humans, as is made evident when Boxer feels distraught after he thinks he killed a human. The irony lies in the fact that the animals assume they are fighting to free themselves from the oppression of human rule. In actuality, they are only solidifying the ruling powers of the pigs.
Throughout the remaining parts of the book, most of the animals keep working harder and harder under the guise that Mr. Jones is going to return if they do not, and that his return would be worse than their current existence. In reality, they are working more diligently to prop Napoleon up as dictator. One by one, the commandments change to help the pigs and other leaders while the lesser animals suffer under even worse oppression than they had experienced under Farmer Jones. All along, the animals assume they are working for a free and independent farm, but the irony is that they are just working towards the pigs becoming more human-like tyrants.