While tooling around looking for references on the Vita Merlini, I came
"An Essay of the Nature and actions of the Subterranean and for the most
part Invisible people, heretofore going under the names of ELVES FAUNS and
FAIRIES or the like among the Low-Country Scots, and termed hubhsisgedh,
caiben, lusbarten, and siotbsudh among the Tramontaines [Highlanders] or
Scottish-Irish, as they are described by those who have the second sight and
now, to occasion further inquiry, collected and compared. "
Robert Kirk (I believe and fully disclose this is yet again from MEMORY) is
the clergyman who one day disappeared into the invisible world, never to
return . . .
This site also has some other articles of interest to Celts, to be taken
with a grain of salt, of course.