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.