Wouldn't Angliana be a better term? Doesn't have the same dubious sectarian associations.

Agreed, "Celtic" is a pretty daft term. There's nothing particularly in common between the different traditions of "Celtic countries" that differentiates them from the musical traditions of their neighbours. Breton music is far closer to French music than it is to Irish music, for example, Gallego music is far closer to Spanish music and so forth.
So far as the similarities between much Irish music and much Scottish music is concerned, "Gaelic" is the appropriate term.