this usual fairy tale is lighthearted and has a happy ending. how does this form provide irony for animal form?

The usual fairy tale is lighthearted? This fairy tale is definitely not that! The purpose of a fairy tale is to approach real life through easily understood symbols. Let's face it, there are many people in this world who've never touched non-fiction and have no desire to do so. What Orwell did with Animal Farm was ingenious. His purpose was to educate the reader, all while entertaining him/her. He intertwines reality with fantasy, hooks his audience and makes a statement. A happy ending would have been complete irony and it wouldn't serve what Orwell intended to do with this novella. Not all fairy-tales have happy endings; the tales you're thinking of in those terms have been re-written and greatly deviated from throughout the years to make them more appealing. Find the original "Little Mermaid," and you'll find out what I'm talking about.

Orwell used this tale to tell the story of Russia under Stalin's rule. There was no happy ending; the sadness and atrocities committed linger long after the last chapter of a story. Stalin's regime isn't studied in great depth. That's what makes this a great addition to the history curriculum in addition to literature. Students are far more acquainted with the Holocaust; can you look at that story and find irony in a happy ending. Use that experience to write your answer to this question.

The fairy-story that succeeds is in fact not a work of fiction at all; or at least no more so than, say, the opening chapters of Genesis. It is a transcription of a view of life into terms of highly simplified symbols, and when it succeeds in its literary purpose, it leaves us with a deep indefinable feeling of truth; and if succeeds also, as Orwell set out to do, in a political as well as an artistic purpose, it leaves us also with a feeling of rebelliousness against the truth revealed. It does so not by adjuring us to rebel, but by the barest economy of plain description that language can achieve; and lest it should be thought guilty of a deliberate appeal to the emotions, it uses for characters not rounded, three-dimensional human beings that develop psychologically though time, but fixed stereotypes, puppets, silhouettes—or animals. [...] In these respects Animal Farm is after all correctly labelled a fairy-story. Its message (which is by no means a moral) is that of all the great fairy-stories: “Life is like that—take it or leave it.”

add the source below to my above answer, and checkout Woodhouse's introduction if it's included in your copy of the book!