False Mermaid by Erin Hart.
We first met Nora Gavin, an American pathologist, in Haunted Ground (2003) when, while lecturing at Dublin's Trinity College, she was asked to investigate a body found in a bog.
Three years later, she decides to return to the United States to investigate something much more personal: the murder of her younger sister Triona. When Triona's murder went unsolved, Nora fled to her homeland to try to escape the fact that she was certain of the identity of her sister's killer but was unable to prove it. She returns to Minneapolis to learn that Triona's widower Peter Hallett is remarrying the sister of his best friend, whom he's known most of his life. Nora is convinced that Peter is an abuser who killed his wife, and is very worried about her niece Elizabeth, now 11.
She reconnects with Frank Cordova, the Minneapolis detective who investigated Triona's case, and who also believed that Peter was the killer, and together they set out to prove Hallett's guilt.
Back in Ireland, Nora's friend and lover Cormac Maguire has his own mystery to deal with. Called to his father's side because of a stroke, he meets a former colleague (now his father's tenant) who is researching selkie mythology. She is convinced that the old folk-tune An Mhaighdean Mhara is based on an actual abused woman named Mary Heaney who was killed by her husband.
Erin Hart's prose is a joy to read, though it's difficult to read slowly enough to appreciate, simply because the parallel mysteries are so compelling. The selkie myth is a favorite, and Hart's treatment of it is not beyond belief.
Readers who have not read Hart's previous Gavin-Maguire books would probably benefit from doing so before reading this one, in order to better understand the characters.