According to David A. Justiss's Dictionary of Monsters, which provides an index of mythical creatures and monsters, a "sedhe" is a fairy. The word sedhe is Gaelic and is pronounced "shee":

They are often very dangerous even if they seem friendly. They are very selective about who they let into their world (and who they let back out), and often have strict rules about not telling anyone where they live or where the entrance to the land of faerie is. The result of breaking their rules is often death or a devastating curse. Some, like the banshee, seem undead and others more like demons than fairies. Even when types of Irish or Europeon fairies seem harmless and helpful, they are afraid of clergy or religious people which usually implies something demonic.

A banshee, though Irish, is not a fairy and is not undead. Nevertheless, David A. Justiss claims that the word "banshee" comes from the Gaelic "Bean-sedhe," which means "woman of the sedhe." A banshee is the ghost of a woman who has drowned her children; this is why she is most frequently seen next to rivers. Seeing a banshee is a "forerunner," meaning an indication of your impending death. It's a common misconceptions that banshees wail; they don't wail, they cry. And if you're seeing the banshee, and she's crying, then she's crying for you, because she knows you're about to die.

So, not only is SEDHE the Spanish Society for the History of Education, but it's also a fairy.