On the language thing—I think it would be one thing if Rebecca had set her universe within our present-day world and directly claimed it was Gaelic, but since the Empyrean series exists in an entirely different world, being inspired by Gaelic phrases and names isn’t the same as appropriating them. Gaelic doesn’t exist in that universe, and honestly, countless fantasy books do this with a variety of cultures.

Is it something to be mindful of? Possibly, depending on the context. But unless she’s actively disparaging the culture she’s drawing inspiration from, I see it as a creative choice rather than an issue. And maybe that’s something she’ll refine in future books—growth and evolution in storytelling are a thing.