You can use "Celtic music" if you like, or any other lumping together terms. The troubles arise from how you use the term. You can use a term like "world music" or "European music" any time you like, and it might be meaningful. But what kind of question would "what are the differences between World Music and European Music" be? Or how would you answer "Are there harmonic differences between European and French music?" Categorisation can produce absurdities if you are not careful, as in your question about differences between Celtic and English.