Irene Scheerer is a Nice Girl. She is also well-off, like Judy, but she doesn't have Judy's great beauty. When Dexter finally realizes that Judy is never going to be his, he turns his attentions to Irene. Her family welcomes Dexter, and he spends many comfortable evenings talking about literature and music with her. Dexter is content with Irene:
[He knew] that Irene would be no more than a curtain spread behind him, a hand moving among gleaming teacups, a voice calling to children (4.17).
It's interesting that in this quote Dexter plainly associates Irene with images of home. She provides a foil for the high romance Dexter experiences with Judy. Irene presents a much more ordinary, boring model of domestic happiness. It's almost as if, knowing he can't have Judy, Dexter looks at Irene and says to himself, "all right. She'll do."
Even though Dexter likes Irene very much, he brutally dumps her with little regret after just one evening spent with Judy Jones. Although he turned to Irene in the first place because he was sick of Judy and her cheating ways, the minute Judy crooks her finger at Dexter again, he goes crawling right back. As the narrator points out,
It was strange that neither when it was over nor a long time afterward did he regret that night.[…] Nor did it matter that by his yielding [to Judy] he subjected himself to a deeper agony in the end and gave serious hurt to Irene Scheerer and to Irene's parents, who had befriended him. There was nothing sufficiently pictorial about Irene's grief to stamp itself on his mind. (5.1)
Irene is so nice, so girl-next-door, that Dexter cannot really feel too much about her sorrows. She does not occupy that romantic, idealistic part of his imagination. So Dexter pities Irene a bit, but because she's not Judy, she doesn't play a huge part, either in Dexter's life or in our story as a whole.