Famous for falling in love with a merman in Loch Lomond while on holiday. When her family forbids her to marry him, she transfigures herself into a haddock and is never seen again (FW).
Family forbids her to marry a merman with whom she has fallen in love (FW).
The most obvious conclusion from the given information is that Plunkett turns herself into a Haddock in order to live in Loch Lomond with the merman. However, the haddock is a saltwater fish and Loch Lomond is a freshwater lake. -BB