Eilis Lacey grows from a nervous, nerdy girl into a confident (but equally nerdy) young woman over the course of Brooklyn. While there are plenty of unplanned detours and unforeseen accidents along the way, this is a classic coming-of-age story if we've ever seen one.
When we first meet her, Eilis is still very much a child on the inside. Her mother is an overbearing lady who often makes her feel "like a child when the doctor would come to the house" (1.186). Her sister Rose tries to help her through this, but that might actually hinder Eilis' growth, as Eilis never actually has to deal with her mom directly since Rose takes the brunt of the damage. Regardless, Eilis' opportunity to go to America (given to her, we must add, by Rose) forces her to do some growing up—and fast.
Of course, it's hardly that simple. Eilis is terrified at first, which makes sense because she's never actually had to make decisions for herself before, but she quickly starts feeling excited. She actually adapts in an interesting way, actively speaking "in a tone that Rose might have used [...] a tone used by a woman in full possession of herself" (1.231). This is some classic "fake it 'til you make it" technique, but little does Eilis know that she's actually started on the path to adulthood.
All of this fun inner turmoil is jacked up to eleven when Eilis arrives in America. Although it takes a few weeks to happen, Eilis is eventually socked in the jaw with one of the most brutal uppercuts of homesickness we've ever seen. We're talking an Ali-level punch, folks—ouch.
Regardless, Eilis mounts her big comeback after serving Christmas dinner at Father Flood's parish, which makes her feel like she could be at "a parish hall anywhere in Ireland on the night of concert" (2.239). Of course, it doesn't hurt that she finds herself an Italian-American sweetheart as well.
Eilis' relationship with Tony ends up defining her time in Brooklyn, for better or worse. That's not to say he's bad—he's a swell dude and Eilis really digs him. Regardless, Eilis is notably hesitant to commit to their relationship, though it's never clear whether this is just nervousness, an indication of a lack of feelings towards him, or just her hesitation to move at a Tony-esque warp speed:
And yet she knew that in his mind Tony was moving faster than she was, and she knew that she would have to slow him down. (3.466)
Either way, Eilis pushes these ambiguities aside and agrees to marry Tony after Rose's sudden death forces her to return to Ireland.
Naturally, this is a trip in every possible meaning of the word. Eilis has grown up a lot since going to America and has become a little Americanized—as she walks one night, "she noticed a woman studying her and she realized with amusement [...] that she must look glamorous in these streets" (4.33). On a more depressing note, however, the fact that Rose isn't there only reminds her of the devastating loss of her sister.
This is amplified by the fact that Mrs. Lacey basically tries to transform Eilis into her deceased older sister. She dresses Eilis in her sis' old clothes like "she was Rose's ghost" (4.102). She gets her a temp job at Rose's old office, which ends with them offering her a full-time job. This really twists Eilis around, leading her to have an affair with Jim Farrell as she wonders whether she should just stay in Ireland. As this relationship grows, she even claims that she doesn't love Tony anymore.
But this is just Eilis' nature. As we've seen time and time again, she has a difficult time holding on to her emotions towards something when it's not directly in front of her. Instead, she becomes content (or acts like it) with whatever's in front of her at a given point in time, which in this case, is the hunky Jim Farrell. Still, we wouldn't be surprised to see this same dynamic happen in reverse and Eilis' feelings towards Jim evaporate when she returns to America.
Although Miss Kelly basically forces Eilis to return to America, we're pretty sure that she would have made that choice either way. That doesn't meant that she's definitely going to end up staying with Tony or anything—we could easily imagine Eilis starting a brand new life for herself when she returns. Her future in Brooklyn might be scary and unknown, but she'd prefer that to the on-rails existence she'd live if she stayed in Ireland.