After the general exposition of Book 1 of Mortal Friends, where we are introduced both to the setting and the main character of Coleman Brady, the "social geography and class consciousness" become more apparent in Book 2.
The social geography is quite specific, actually: it is the interactions and dealings of the Irish in Boston, Massachusetts in the mid 20th century. We now get to see both Coleman Brady and his son in a "modern" and "American" setting (instead of the antiquated setting of Ireland). What is interesting about the social geography here is that Brady hobnobs with some of the main historical figures of the time, even the Kennedys. (Some scholars consider this both a distraction and a flaw.) Brady brushes both with an interesting mayor of Boston as well as characters from the Mafia. Through these social interactions, Brady becomes more and more introspective about his own life and thoughts about being Irish in Boston. We learn a lot about the social geography through Brady's actual thoughts (which, in my opinion, are conveyed through what borders on a stream of consciousness technique). Any time Brady says he is "sure" about something, we can be sure (as readers) that he is most definitely NOT sure.
In regard to class consciousness, it is the unity of the American Irish in the middle class that is the most important element. What is neat about this book is that the upper class politics of Boston are not the most important element, as they are in many other novels! In fact, the Bradys are a middle class family! Truly, there is some abrasiveness in regard to class. During the novel, Brady has to learn a balance, especially in regard to his middle class family. For example, Brady expresses this idea in a comment about his grandson:
I finally learned to love someone without eating him.
Even with this class separation (between the often abrasive middle class and the more calm and smooth upper class), the Bradys still interact with important people in politics: the Kennedys, the Mafia, the local leaders (mayors, etc.). Everyone, including Irish Americans, can transcend their class, then, and enter into the true history of the time through their interactions.