I'm cringing. Runes are not Celtic. Seriously. Runes are 100% Germanic. As far as is known, the Celts did not develop a writing system of their own, preferring an oral tradition. When they needed to write something down, they borrowed the Latin alphabet. Even ogham, the stone inscription alphabet that was used exclusively for grave and boundary markers, was a fairly recent invention, traced only as far back as the 2nd century CE, if that early.
Regarding things Celtic, you'd be wise to consider anything that's not published by a university press to be at least 50% BS. There's a lot of crap, fantasy, and misinformation out there. Tread carefully. You can find good info about Celts, but you have to dig for it.