The biggest example of irony in "The Lottery" is the title itself. Most people associate a lottery with drawing for a prize of some sort. The brutal nature of the ritual and its acceptence in the town is in direct odds with calling it such a innocuous name. Since the person who "wins" the lottery is stoned to death, people are truly hoping not to win, also in opposition to most lotteries where people try to win. Even the ritual itself is set up like a town fair, with everyone gathering and then going home for dinner; the children participate, and no one thinks anything is wrong with it.
Another example of irony in Shirley Jackson's story which may not have been mentioned because it is not conspicuous is the fact that this deadly lottery is taking place in a part of the American Midwest and at a time when the vast majority of the population were devout Christians and regular churchgoers. Naturally they would have been taught since childhood that one of the Ten Commandments was "Thou shalt not kill" and that they should love their neighbors as themselves and do unto others as they would like others to do unto them. And they may have believed in these teachings and tried to abide by them for 364 days of the year--but on that one day in June they murdered one of their own neighbors in a most terrible fashion. The story has nothing to say about churches or church attendance; otherwise the storm of protest that followed its publication in The New Yorker would have seemed trivial in comparison to the reaction that would have otherwise occurred.