This discussion concerns the portrayal of two supernatural beings, the mermaid and the banshee, within Irish and Scottish Gaelic folk tradition. Despite some obvious differences, such as appearance and habitat, a comparison reveals many underlying similarities, allowing mermaids to appear in narratives typically associated with banshees and vice versa. It is argued that beliefs and narratives surrounding both beings reflect a common cluster of ideas and anxieties regarding the status of women within exogamous and patrilocal families. Further connections are drawn between folkloric accounts and portrayals of female death-messengers and sovereignty-figures in early Irish literature.