Daisy's statement is ironic because the home she and Tom share is not one based on love or even convenience, but out of a duty to their upper social class. The type of irony being discussed here can easily be defined as "the opposite of what's expected." Daisy's comments that "a home influence" will be good for Jordan, it comes with the suggestion that the Buchanan household is a happy one (whatever that means) and will make Jordan a better person at the end. But the reader quickly learns the opposite is true.
In Chapter 1, Jordan tells Nick of Tom's infidelities and how he's "got some woman in New York." Then, with Daisy seemingly unmasking her public persona, she reveals to Nick that she hopes her daughter will "be a fool—that's the best thing a girl can be in this world, a beautiful little fool."
To really see the sickness of the Buchanan household, it's important to look at the home after all the tragedies in the novel take place. With both Daisy's and Tom's lovers dead, the two return to one another and leave on a vacation. They don't attempt to clean up the mess they made. Instead, "they retreated back into their money or their vast carelessness..."
Jordan's decision on the night of Myrtle's death to go with the Buchanans instead of with Nick is a symbolic one. Unfortunately for her, the Buchanans' home life turned out to be such a bad influence on her that she basically became one of them.