The irony is addressed by Orwell in Chapter 7 when he writes, "All that year the animals worked like slaves. But they were happy in their work; they grudged no effort or sacrifice, well aware that everything that they did was for the benefit of themselves and those of their kind who would come after them, and not for a pack of idle, thieving human beings."
It is true that they are not working for a pack of idle, thieving human beings. Instead they are working for a pack of idle, thieving pigs. The pigs have moved into the farm house and started trading with neighboring farms. When some of the animals remember the commandment that "no animal shall sleep in a bed", the commandment is simply changed to "no animal shall sleep in a bed with sheets." Squealer assures the animals that all the sheets have been removed. So the work continues and the animals "volunteer" to work up to 60 hours a week. Of course, if they don't "volunteer" their food rations are cut in half. Thus, the animals are coerced into the "joy of service" by the need for sheer survival.
Part of the irony is where it comes from. Squealer is the mouthpiece for the pigs, often compared to "Pravda" in the Soviet Union. The irony is simple: he and the pigs speak eloquently of the "joy of service" and the "dignity of work," but they don't DO ANY. The speak glowingly of the service and sacrifice of others, and enjoy the fruits of their labors, all the while contributing nothing themselves. It's interesting that because he does nothing, Squealer just gets fatter and fatter until his eyes are just slits by the end of the story. The other animals work themselves to death; the pigs eat/drink themselves to death. No big surprise here ....