The usual story is that the Mary Chinidh was trapped on land because her husband to be had stolen her mermaid tail (or her seal skin - the sea maiden can be a mermaid or a seal who has taken human form). When she finally finds her missing tail/skin she returns to the sea even when it means leaving her children behind. I don't know the version Angun cited about the mermaid's crown, but the tale is international.

Scholars have suggested that the name "Mary Chinidh" (Mary Kenny) may derive from the name Mélusine in related European mythology.(when I referred in the previous paragraph to the tale being international, I had in mind stories more closely parallel with the background story of An Mhaighdean Mhara, however).