In my comments on Tana French's novel The Likeness, I said that the core psychological exploration in the novel were sound, but the mystery itself was very contrived, and French didn't seem that interested in the actual plot of the novel; Faithful Place is much more successful at integrating the psychological themes with the plot.
Detective Frank Mackie left home 20+ years ago, and hasn't spoken to anyone in his family except one sister in all that time. But he returns when a suitcase belonging to his girlfriend, who disappeared 20 years ago, is discovered. Frank is torn between wanting to find out what happened to her and trying to stay away from his abusive parents. As one expects in this sort of novel, there are family secrets long buried which come to the surface.
I think French does a great job of working her theme of family secrets through the various threads of the novel. Frank has been hiding his background from his ex-wife, she has been hiding the fact that she's introduced their daughter to Frank's family, and so on.
Since French's theme is the exposure of these secrets and the effect they have, it makes sense that she solves the mystery of what happened to Frank's girlfriend long before the end of the book. The point of the novel is about the aftermath when the secrets are exposed; by getting there "early", she gives herself time to explore the results in the detail they need. (This is another improvement over The Likeness, where the resolution is placed more traditionally close to the end, and so the ending feels rushed).
As a side note, this seemed to me to be French's most optimistic book, and I enjoyed that as well.