Dramatic Irony is used because the reader knows that Napoleon is slowly taking over the farm but the animals don't.
We will work harder, Napoleon is always right.
As each commandment is changed the animals think they remember it like it was before but they aren't sure. We as the reader know that the commandments are being changed each day.
I have decided upon a new policy......
Dramatic irony is used because the sheep live by the commandments and they restate the rule daily but what the animals do not know is that the commandments are totally changing and those rules don't apply as much as they used to.