The story of An Mhaighdean Mhara in Irish and in Scottish Gaelic usually includes the theft and hiding of either her tail (mermaid) or her pelt (seal). And she usually leaves her human children behind (though I did read a version of the tale in which she tried to bring them to the sea - I think that was one of the stories in the schools collection of the Irish Folklore Commission, 1937-39; see https://www.duchas.ie/en/info/cbe) The Scottish song, Òran Na Maighdinn-mhara, has a line in the chorus, "'S ann le foill a mheall thu mi", it's with guile/treachery that you deceived me.

Mary Chinidh does shift-shape; she is either a mermaid or a seal who has taken on a human form, but who returns to being a sea creature when she gets the chance.