The novel ends with Tom looking at the barn on his property. He remembers the first time he went to the barn with Birdy and how the Widow Breen had pointed her shotgun at him for nosing around:
The recollection was so vivid that he turned quickly to their own kitchen porch to be sure she was not standing on it, but she wasn't. It wasn't her barn now. It was his—absolutely. He didn't have to go around like a low-down Dolan any more. It came suddenly into his mind that the Widow Breen had told that to him long ago. (63.31)
Memory and the past are big issues in the novel (check out our "Themes" section), and these final lines emphasize the way that the past can feel like a very real force in the present. Tom even has to check over his shoulder to make sure a moment from the past is, in fact, passed. Which (duh) it is.
But the power of the past is still there: the Widow Breen isn't pointing a shotgun at Tom, but she did point him toward his future. Tom and his family aren't weighed down anymore by the shame and hurt of their no-good relatives. That's in the past, too.