I'm an individual who needs closure, for everything, as painful as it may be. Yet, for this film, I think it's better, for us the audience, to not exactly see or hear what happens next.
In reality, Tommy gets court-martialed and goes to jail, Brendon pays off his debt, Paddy goes back home alone, both Tommy and Brendon have a long ways to go in repairing their relationship, etc. Considering what followed though, I assume the general audience was spared from a far more depressing conclusion.
The director mentioned that he did not want to end the movie with a 'happy' ending, instead he wanted an open and 'realistic' ending.