Werewolves don't exist in old Celtic beliefs. Celtic faith doesn't contain animist beliefs. Werewolves are Germanic in origin (Saxon in particular, if I'm remembering correctly) and since colonization, the concept of Werewolf has been influenced by Native American animism and some versions of the legend of the Wendigo in particular.
Norse is a possibility, or Saxon or any number of other cultures that used alphabets composed of linear conflagration. There really is no historical accuracy to the idea the the Celts used a Runic alphabet. There's evidence to suggest that they borrowed the Greek alphabet. Ogham, the only known actual Celtic alphabet, has only been found in association with grave sites/markers and bears no resemblance to what most people would call Runes. Those symbols could've just been a Hodge-podge and meaningless. Or they could've been a recipe for chicken noodle soup. I'd have to see them to tell.
Runes have nothing to do with Werewolves. It's actually pretty compelling evidence that the whole thing is a hoax being put on by someone just slapping as much "occultism" as possible to incite superstition. Kinda like the Satanic Cult scare of the 1980's that turned out to be nothing but scare-mongering.