Look it up on Wikipedia (I know, not always a reliable source, but I checked and in this instance it's good). Search for 'ogham' and 'futhark', read the first couple of paragraphs of each, then compare the illustrations of both alphabets to the script you have used here. While it doesn't exactly match either, it's clearly closely related to the Norse characters, while not really resembling the Celtic ogham alphabet.
The Celts very likely adopted the Norse alphabet because they didn't have one of their own. It's like referring to our own alphabet as 'English letters' - they're not. We adopted our letters from the Romans and our numbers from Arabic. Likewise, while the early Celts would very probably have known and used the futhark script, they didn't create it and it's misleading to describe it as Celtic. It's just their variant or dialect of an already-existing alphabet.
But I got it from the Horrible Histories book, Cut-throat Celts.
However, +1 anyway, for promoting obscure knowledge (even if not quite accurate!)
That's not Celtic. That is some variant of the futhark, the Old Norse runic alphabet. Runes were a major influence and possible origin of the Ogham script, AFAIK the earliest distinctively and definitely Celtic alphabet, which however differs significantly from the Norse alphabet. But the characters you have here predate any written Celtic language by several centuries.