How and why is the sentence '"The importance of keeping the pigs in good health was all too obvious" an example of Orwell's use of irony?=)
The sentence assumes that it is accepted that the pigs are important, in fact, that they are more important than any other creatures. The words "all too obvious" says that there is absolutley no doubt on the part of the speaker (or narrator) that something is so. In this case, the "something" is that the pigs are essential and the "so" is that because the pigs are essential, keeping them healthy is necessary. The wording of the sentence says that anyone would clearly understand this idea because it is a fact, pure and simple. The irony then, is that the whole purpose of the animals' establishing Animal Farm was to create a place where all the animals were equal. The last and most important of the seven commandments of Animal Farm states: "All animals are equal." The sentence you asked about contradicts this idea completely. The sentence says that the pigs are more important thus giving the opposite meaning. This shows how much has changed since the beginning of the story when the commandments were first written. It shows that the story has come full-circle. The animals are back where they started with a tyrannical ruler (Mr. Jones) now simply replaced by Napoleon.
Another irony I find in the line is that, given the goals of socialism, the equality of all people (animals), there is absolutelly no reason why the health of pigs is any more important than the health of any of the other animals. Of course, the irony is between what is said and what is meant. This is a hint that the pigs are going to be something "special" on the farm, that the dream of the Old Major is going to come to naught, that the situation of the other animals will not improve, but only move from one sorry situation to another.