How is the initial rebellion in George Orwell's Animal Farm ironic in light of its ending?

The animals end up working harder under the pigs than under the people. 

Irony is when the opposite of what you expect happens.  Old Major shares a vision of a perfect world, an animal utopia where the animals are only beholden to themselves and live a life of comfort and leisure.  They assume that after they oust the humans, such a dream can be realized. 

When Jones and his men get drunk and don’t feed or take care of the animals, they rebel and the humans are run off the farm.  The animals expect that everything will be wonderful now that the humans are gone.  They make plans to change the way the farm is run, so that the animals can live comfortably and take care of everything on the farm themselves. 

The first sign that this is not to be is when the pigs take charge.  They immediately begin appointing special privileges to themselves, including special food like the milk and apples.  Squealer, the pigs’ mouthpiece, has a justification for every liberty the pigs take. 

"You do not imagine, I hope, that we pigs are doing this in a spirit of selfishness and privilege? Many of us actually dislike milk and apples. I dislike them myself. … Milk and apples (this has been proved by Science, comrades) contain substances absolutely necessary to the well−being of a pig. (Ch. 3) 

The pigs even begin wearing clothes and sleeping in the house.  They learn to walk on two legs.  Napoleon also secretly trains a special force of guard dogs.  The other animals are forced to work more and more.  There are no days off and retirements like promised. 

Throughout the spring and summer they worked a sixty−hour week, and in August Napoleon announced that there would be work on Sunday afternoons as well. This work was strictly voluntary, but any animal who absented himself from it would have his rations reduced by half. (Ch. 6) 

Animals who do not go along with things are run off, like Snowball, or mysteriously confess to imagined wrongdoing and are then killed.  With all of the scapegoats, the animals are living in fear.  No one has the will or initiative to oppose Napoleon and his minions.  This is ironic, of course, because the animals thought that when they ousted the humans they would be happy on their own, but the other animals were even more abusive to them.