Pah! Who were these Celts anyway?
Just a load of disparate, warring tribes possibly connected only by some linguistic similarities who arrived on these islands around 500 years BC and caused mayhem and a lot of noise till the Romans arrived and sorted them out.
And no, they didn't invent the bouzouki.
Nowadays the word has such negative musical connotations that I believe it's far better and safer to name each musical tradition according to its geographical origins.