The Brides of Rollrock Island is based upon selkie mythology. Women who come from seals are a great lure to the men of the island and often commit adultery, leave their wives or are generally entranced by them. A witch is paid to bring the women out of the seals, women who are never happy on land and eventually succumb to the desire to go back "home". Overall the myth and the book is entirely strange.
The book starts off on a bad foot with a chapter involving children who are frightened of the witch Misskaella. The children speak a bit strangely and the entire chapter is written oddly. Unfortunately, this entire chapter could be completely omitted and you would not lose a bit of story, although it would save you some confusion. The next section of the book follows Misskaella, the future witch who brings women out of the seals for money, from childhood to old womanhood. This was by far the most interesting section of the book as it established the selkie lore and the role Misskaella played within it. Misskaella, however, is not a likable character, but none of the characters in the book can hold such a title.
After the section in which Misskaella grows and becomes the witch, the sections vary viewpoints further with those impacted by the seal women. However, one aspect never given is that of an actual seal woman, why is that? But overall, I felt these sections added little to nothing to the story especially as there is no actual climax to the story. Aside from Misskaella, the characters were even more dull and offered nothing to extend the lore or novel overall. The section surrounding Misskaella could have easily been a short story and left at that.
The writing of Lanagan is interesting but at times too wordy and often lacks purpose. Sentence after sentence in the novel would offer nothing of use for characterization, worldbuilding, mythical lore or anything else of worth.
Overall Brides of Rollrock Island will appeal to some readers, although who exactly would be hard to guess. But for my personal taste I felt the book was too meandering and odd to hold my interest.