The word "spontaneous" means unplanned, something that happens without forethought or organization, something that develops as an idea that comes into being without assistance.
When Napoleon gives orders that "spontaneous demonstrations" should begin and should be held weekly, he is contradicting every part of that definition. The demonstrations are not unplanned - everyone knows that they will happen at the announced time every week. They are organized - the animals know the order in which they will march, the types of slogans they will shout as they march, and what else will happen during the demonstrations. The demonstrations would not have started without Napoleon ordering them - the animals were too busy working, trying to raise food enough for everyone on the farm, to spend energy on something as nonproductive as the demonstrations unless they were forced to do so.
Irony is defined as "the literal meaning of a word or statement is the opposite of that intended." This is certainly the case here. Nothing about the "spontaneous demonstrations" was actually "spontaneous."