Whose story is being told, Irene's or Clare's?

In short, both character's stories are being told. Irene and Clare are two sides of the same coin, and neither would have much of a story without the other to serve as a foil. Irene's obsession with familiarity, security, and tradition is set off by Clare's risk-taking, passion, and desire for change, and vice versa. The novel tells the story of their intertwined lives and how they drive each other's actions.
It could be argued that as Irene is the (third-person) narrator of the story, the story is hers, but alternatively, she could be the lens through which we watch Clare's story unfold. Either argument is viable if a choice must be made, but most accurately, Irene Redfield and Clare Kendry are two parts of the same story.