You are refering to Chapter 22 in which Holden speaks to Phoebe having gone into his parents' home and waking her up to talk to her. I suppose the irony of this quote from Phoebe when she tells her brother off for swearing so much lies in the fact that Holden has spent the entire novel up until this stage engaged in swearing repeatedly. Clearly, Phoebe is unaware of this, but we know as readers that Holden is not somebody who feels that there is any problem with swearing, and so the irony lies in the way that Phoebe tells her brother to do something that he is clearly incapable of doing or that he obviously sees no reason to do. Of course, the irony is shown when in a matter of a few paragraphs Holden is back to swearing with his customary panache, though he does choose to swear more in his own internal thoughts than openly so that Phoebe can hear him.