The overall tone of Emma is ironic, but within that, it is also sympathetic and comic. At the end of the novel, the tone is romantic.
Emma is filled with situational irony (which is when events works out to be the opposite of what we expect), as well as dramatic irony (in which the audience knows what the characters in a story don't), and verbal irony (in which people say things that are the opposite of what they mean).
Emma shows events through the clueless Emma's eyes, so we have a good deal of situational irony. The chief example is Emma entirely missing the secret love between Jane and Frank, despite all the clues Austen sprinkles—and we as an audience are deceived as well. Ironically, Emma thinks Frank looks down on Jane, when the opposite is true.
Dramatic irony occurs when we know that Mr. Elton is after Emma as a bride, while she is convinced he is after Harriet. Verbal irony is sprinkled throughout the novel, but the opening provides a good example: Mr. Woodhouse mourns Miss Taylor, the governess, getting married, when really in that society, a governess making a good marriage was the equivalent of hitting the lottery.
Instances of verbal irony lead to much of the book's comedy. An audience of Austen's time would have been laughing, for example, at Mr. Woodhouse's pity of Miss Taylor's new state, just as we would laugh today at a character in a novel pitying a person who had a great stroke of good fortune.
Austen shows sympathy for all of her main female characters, with the exception of Mrs. Elton. She is sympathetic toward Harriet, Jane, Emma, and even Miss Bates, having Mr. Knightley chide Emma for making a cruel joke at her expense, After all, as Mr. Knightley points out, Miss Bates is a poor spinster who has lost social status.
The happy resolutions and pairings at the end lend a romantic and joyful tone to the novel's final chapters.