On one level it's ironic because it says one thing and means another. On another level, I think Napoleon (Orwell) is telling us that it's impossible to be free as long as we allow for leaders like Napoleon who have no regard for the truth, who create reality to fit his (their) needs/wants.
There is an old Gospel saying that fits in well here: "The truth shall make you free" (John 8:32). If you have no access to the truth, then you will never be free ... no matter how frequently and earnestly your leaders tell you that you are.
The statement by Napoleon, in "Animal Farm," saying that all the animals are free is ironic because the animals are not free. Most of the animals are in the same situation they were in when Mr. Jones ran the farm. The animals are still overworked and poorly fed. They are still ordered about by others; the only difference being, now the "others" are also animals.
Napoleon has the animals thinking they are working for a goal of their own design rather than the idea or goals of the humans. The master-mind, Napoleon, is very charismatic and easily persuades most of the animals that what they are doing will only advance them to being freer than they were under the hand of humans. As the novel progresses, Napoleon and the other pigs become more and more human-like. They even begin trading with humans for the things the other animals can't make. One example is alcohol.
"Winter is severe and the rations are reduced, except for the pigs and the dogs. Squealer calls it a “readjustment.” He tells the hungry animals that reducing everyone’s rations would be against the fundamental principles of Animalism."
" Orwell challenges the Soviet state's—and any totalitarian state's—method of controlling public opinion by manipulating the truth and, in particular, rewriting history."