Good crime novels treat you to a good story that makes you delighted to get to the end and find out what happens. Great crime novels create a sort of poetry that puts you into a new world you don't want to leave no matter how horrible that world is. That is what French accomplishes.
Take this scene early on. There are two detectives who have grown up and mostly worked in the poorest, most vile parts of Dublin slums. It's really all they know. But by random chance they get called out to interview people at a very wealthy boarding school for girls. It's a place that is completely outside of their experience. Here is how one of the detectives describes walking up to that school:
"Maybe I should have hated it. Community school me, classes in run-down prefabs, keep your coat on when the heating went every winter, arrange the geography posters to cover the mould patches, dare each other to touch the dead rats in the jacks. Maybe I should have looked at that school and wanted to take a shite in the portico.
It was beautiful. I love beautiful; always have. I never saw why I should hate what I wish I had. Love it harder. Work your way closer. Clasp your hands around it tighter. Til you find a way to make it yours."
If you don't want to find out where that character will lead you, I don't understand you.
Later on we see things from the point of view of the girls themselves. And let me tell you, boarding school can be it's own form of hell. At one point we get a full page of interior monologue from a girl about all the things there are in the world for her to be scared of and how society wants and needs a "good girl" to be scared of these things. It's heartbreaking.
The novel alternates between the points of view of the detectives and the girls and both sides have their own interior horrors. But both sides also see a chance at hope and redemption, even though they've all made horrible, horrible decisions.
The real brilliance here is not showing that life can be just as terrible for rich teenaged girls as it can be for poor ones (though that is done well) it's that it shows the power of friendship, the dangers of looking too close at things and that secrets can be the cruelest things of all.