I dunno... "Shallan" looks like one of those created names to me, although maybe it does have some Celtic background to it. Maybe Daividh or Phyllis might know, as they're both really into the Celtic stuff.
I did discover that a "shallon", however, is a species of evergreen shrub discovered by Lewis and Clark on their famous expedition into the wilds of North America. The Indians' name for the fruit of this shrub was "salal", and Lewis changed that word to "shallon" to refer to the entire plant.
Here's where you can see a picture of the shrub:
http://www.uky.edu/ArtsSciences/ModernStudies/HumSocSci/lc94/Section2/PhaseX/Scientific/Botany/Botany.html
-- Nanaea