What is ironic about him calling the others a “pack of young fools"?
First, let's look at the context. The words "pack of young fools" come from the mouth of Old Man Warner. He is speaking to his friend, Mr. Adams, who said that some towns are thinking of giving up the lottery. At this comment, Old Man Warner snorts and goes into a tirade about young people and their thoughtlessness. Here is the quote:
Old Man Warner snorted. "Pack of crazy fools," he said. "Listening to the young folks, nothing's good enough for them. Next thing you know, they'll be wanting to go back to living in caves, nobody work any more, live hat way for a while. Used to be a saying about 'Lottery in June, corn be heavy soon.' First thing you know, we'd all be eating stewed chickweed and acorns. There's always been a lottery," he added petulantly. "Bad enough to see young Joe Summers up there joking with everybody."
The irony here is that Old Man Warner is the fool. He thinks that the lottery will bring a good harvest. There is no logic here. It is pure superstition in the worst sense, especially as a person is being sacrificed in the process. From this perspective, the only fool here is one who believes in the lottery. Change for the sake of change might be bad, but when change is for the better, change is wise. Not to change is foolishness.