And then it occurred to her that she was already feeling that she would need to remember this room, her sister, this scene, as though from a distance. (1.186)
Eilis is so nostalgic that she starts missing the past while it's still the present. Still, given that she'll be halfway across the planet (in an era, we must add, that you couldn't just use Snapchat or Skype to keep in touch), it's understandable that she's hesitant to leave the only home she's ever known.
What she would need to do in the days before she left and on the morning of her departure was smile, so that they would remember her smiling. (1.225)
Eilis is concerned with presenting a happy face so her family remembers her at her best. That's really thoughtful, and it shows that Eilis has a maturity well beyond her years. No matter how mature she might be, however, she's still totally unprepared for how this journey will change her as a person.
For the past few weeks, she realized, she had not really thought of home. The town had come to her in flashing pictures [...] but her own life [...] she had kept out of her mind. (2.54)
Interestingly, Eilis isn't actually homesick at first. Although she occasionally thinks about Ireland, she never dwells on her memories long enough to get truly bummed out. That's totally an effective strategy but, as we see here, it can only work for so long before all those memories start flooding back. And what's she supposed to do when that happens?
Miss Keegan said that it was not really Christmas if you were not in your own house in Ireland, and she was going to be sad all day. (2.184)
This is a frightening thought. If Miss Keegan, who's been in America for some time, still can't fully shake her nostalgia for her home, then what hope does Eilis have? As it turns out, the trick is to make new memories that hold as much meaning as the old ones—like helping out Father Flood at the parish Christmas party.
Eilis read the letter a few times and [...] she could hear Jack's voice in the words he wrote, she could feel him in the room with her. (3.787)
Although Eilis can ignore her memories when she's caught up in the hustle-and-bustle of Brooklyn life, her family's letters bring them all back in full force. This can be overwhelming for Eilis, and ends up causing her a great deal of emotional anguish.
It was as though Rose's death had happened long ago, and her night with Tony remained with her as something powerful, still present. (3.827)
Now this is what we were talking about when we suggested that Eilis should create new memories. Although Eilis is never quite sure if she made the right decision by doing the hanky-panky with Tony, it does comfort her on some level because it shows that life continues, even in the face of tragedy.
Eilis concentrated on Rose's spirit and tried to keep her mind from dwelling on what was happening to Rose's body just beneath them in the damp clay. (4.41)
In other words, Eilis is trying to think of Rose as she was at her best, not how she is now. We're sure big sis would have wanted it that way, too. After all, Rose did a lot for her sister, and it's only fitting that Rose be remembered as she truly was—classy, talented, and a monster on the golf course.
In the morning it was hard not to think that she was Rose's ghost, being fed and spoken to in the same way at the same time by her mother. (4.102)
Unable to fully accept her eldest daughter's death, Mrs. Lacey tries to make Rose's memory live on by turning Eilis into a walking, talking wax replica of her deceased older sister. It's a little creepy—like Mrs. Lacey is a psychic trying to channel a spirit.
And not only that, but everything else that had happened in Brooklyn seemed as though it had almost dissolved and was no longer richly present for her. (4.190)
Now that she's returned to Ireland, Eilis is put in a strange position—it's Brooklyn that's in her past. Funny how things work out, huh? In fact, it almost feels like she never left Ireland in the first place, like it was just one really long (and insanely detailed) dream that she's just now waking up from.
Some time in the future, she thought, she would look at them and remember what would soon, she knew now, seem like a strange, hazy dream to her. (4.366)
Ultimately, Eilis lets go of her experiences in Ireland (and her brief romance with Jim Farrell) because she knows that it's not the direction she wants her life to go in. That doesn't mean she'll necessarily look back with regret—instead, she'll treasure the memories on their own merit.