Edgar B. Herwick III, of the National Public Radio station WGBH in my erstwhile stomping grounds of Greater Boston, got me on the phone (Skype, actually) to find out why Celtic music is “keltik” but the Boston Celtics are “seltik.” Of course, I told him. You can listen to the radio show, or read the script, or both:
Why We Pronounce ‘Celtic’ Music And Boston ‘Celtics’ Differently
Sesquiotica 
This is really interesting, thank you! I’m a bit confused about one thing: if the French influence didn’t extend to Ireland, Scotland, and Wales, then why did the Scottish team Celtic get pronounced with the soft c?
The French influence didn’t extend to the Celtic languages, but it did extend to English, and English extended its coverage to Scotland (and Wales and Ireland). The English dominance in the late 1800s brooked no impertinence from subaltern languages such as Gaelic. English was English and Gaelic was Gaelic. So soft-C Celtic it was.
Thanks.