Joe F, Mr Google has plenty to say on the subject (and of Mermaids, Sirens, Water Sprites etc, in general) and that fount of knowledge, WIKI, says :

"Lorelei is a feminine water spirit, similar to a siren or mermaid, said to send sailors to their death, by luring them near cliffs with her beautiful singing voice."
Close enough for me.

Of course, there is that theory that sailors claiming to see Mermaids were probably seeing Dugongs or Manatees (aka sea cows).

Anyone have any songs about these mammals???

R-J