I read this after reading her most recent book, Normal People. I wish I'd read this first, as it's a slightly less refined version of her very particular style of storytelling, which is not to say that it's not enormously enjoyable and satisfying in its own right.
It's set mostly in Dublin, and tells the story of a young creative couple, both women, who befriend a slightly older, successful couple, and begin a complex menage-a-quatre of sorts. Rooney is tremendously good at chronicling emotions in relationships and friendships, with brilliant dialogue and economical descriptions of settings, weather, houses etc. It's funny and sharp, sexy and dynamic. I loved it.