If you've ever watched The Real World, then you know that a full house of roommates inevitably spells drama. Mrs. Kehoe's boarding house is no exception to this rule.
First you've got Patty and Diane: the cool girls. In many ways, they're foils to Eilis' friends Annette and Nancy back home, as both pairs provide a striking contrast for Eilis' nerdy demeanor. Patty and Diane love to have fun and go wild at parties, which inspires Eilis to dress like them in an attempt to "feel more like the girls she had seen dancing" (3.12). Eilis desperately wants to be like them, but she can't—not yet.
That's because she's first stuck with Sheila and Ms. McAdam, who are way less fun. They're snooty. They're boring. They're judgmental. All in all, not a winning combo. Because Eilis is socially awkward by nature, it takes her awhile to dig these two ladies' claws out of her back, but she eventually builds up the courage to tell them off, calling them "'wallflowers [...] with [...] sour look on their faces'" (3.409). Ooh. Burn.
This is an important moment for Eilis. Not only does it represent her embracing her youth, but it also represents her standing up for herself, what she believes in, and who she wants to become.
Annette and Nancy are a pair of lovely Irish lasses who also happen to be Eilis' childhood friends.
In many ways, Annette and Nancy are foils for Eilis' Brooklyn roommates, Patty and Diane. Like Patty and Diane, Annette and Nancy are a bit more outgoing than Eilis, more willing to throw caution to the wind and get all makey-outy with the first boy they see. Of course, there are plenty of differences too, but they mostly relate to the cultural gulf between small town Ireland and big city America.
These differences are placed into high relief when Eilis returns to Ireland. You can see this in their respective weddings: Nancy has a big, traditional ceremony, while Eilis marries Tony in a courtroom. That's a wider gulf than the Atlantic. For her part, Nancy can see this, saying that "'everything about [Eilis] is different'" (4.183). Well, she's right—Eilis is different, and that's why she ultimately must return to Brooklyn.