The best option seems to be "no animal had ever retired".
Situational irony is a literary device consisting of leading the reader to believe a certain event / ending will take place and then unexpectedly changing the outcome.
In Animal Farm, the society ruled by animals and for animals was supposed to be one of equality and fairness. However, as soon as the pigs - the rulers of the Animal Farm - have a taste of power, they deceive the other animals into working for their own well-being, as opposed to working for everybody's well-being. In this excerpt, the situational irony seems to come from the fact that the animals - and the reader - believe they will retire at a certain a age, that a nice place will be set aside for them to rest after having worked so hard. But, as is revealed by the sentence "no animal had after retired", the outcome is not what was expected.