This novel takes place in Brooklyn. It's also called Brooklyn. What a crazy coinkydink.
But like most things in the magical world of Lit, this novel isn't just titled after its setting. Brooklyn means a variety of things for Eilis throughout her coming-of-age journey. At the beginning, it's a bizarre, foreign land. When she arrives, it's symbolic of everything that Ireland isn't, which makes her long for home. Eventually it becomes a second homeland… and then, when she returns to Ireland, she realizes that it has replaced her hometown as her real home.
Like other geographically-titled books (think Middlemarch, think Wuthering Heights) Brooklyn is about the ways that the culture of a specific location changes a life. For Eilis, she gets thrown headfirst into the melting pot… and realizes that it suits her just fine.