I saw a facebook ap recently called “Celtic Runes”. A friendly note from a frustrated medievalist: I know runes are all mysterious and such (a PSA for another day), and so are the Celts (ditto), but there is no such thing as a Celtic rune.
The system of writing used in Ireland (which, I assume, were the Celts Facebook had in mind) was Ogham, which is actually really cool. It uses strokes oriented around a medial line:
Since this alphabet was mostly used for inscriptions on stone, the medial line was usually the edge of a monument.
Runes were used by continental German tribes, the Norse (so in Denmark, Norway, Sweden (?), Iceland, and Greenland mostly), and Anglo-Saxon England. They could have magical associations, but they were mostly just a normal alphabet for writing a language (Old Norse or Old English, as the case may be). This is the Old English one: