The irony of "spontaneous demonstrations" lies in the fact that Napoleon was ordering them to take place. "Spontaneous" demonstrations break out on their own. They are not planned, necessarily, and they certainly are not prescribed by senior government officials—and that is what Napoleon is. So in chapter 9, when Napoleon orders a "spontaneous demonstration" to be held each week, the situation is rich with irony.
The entire thing is full of planning and regimentation on the part of the pigs, who order that the animals engage in a military-style procession while the horses carry a banner reading "Long Live Comrade Napoleon." Fittingly, the sheep especially enjoy these events—they bleat out anyone who dares to complain, however quietly, about having to attend them. But the point is that these supposed shows of support for Animal Farm are forced, rather than voluntary. This does not mean, at this point, that they are inauthentic—in fact that is one of Orwell's key points in the book. The phrase "spontaneous demonstration" is used to illustrate how those in power can manipulate language through neologisms, oxymorons, and other devices to cast doubt on the very possibility of objective truth.
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."