Here's a link to an Enya interivew:
It's in Spanish, but it has some interesting thoughts; so I'll translate some of it for you. The interview was taken just after finishing Amarantine
interviewer: Where do you get your inspiration?
Enya: It can vary so much. When I compose, I feel that the ideal is not to impose anything on myself and not to imitate myself; this way I feel at liberty to let the melody develop as it will, without thinking about the time it will take to evolve. Only when a melody is finished, I listen back to it and ask myself, "Where is it coming from? What do I want to say?" It can be a very visual thing for me, or an emotional feeling, or a combination. It is difficult to say what are the primary influences.
I think what's important is that the inspiration comes "taking a stroll in the park", so to speak, and out of everywhere. It can be a stroll on the beach; good times ... it can be many things. Everything evolves into what you want to say.
interviewer How do you get your sound?
Enya I think the way of arranging ... I always found myself. Your voice is your voice; I can't change that. Whatever song it may be, when I sing it, it sounds like me [Enya proceeds to describe how she varies her voice according to the song].
I would say I gear towards classical music. I am a great admirer of this sort of music, and it is a great influence for me. My arrangements, with all the string pizzicatos , come from classical music.
interviewer Tells us about your experience doing the music to Lord of the Rings
Enya It was just ... a marvellous experience, having read the book just a few years ago, before I'd heard they were filming a movie to it. It's a complex book, so I thought it'd be hard for them to capture the essence of the novel; but when I saw the first movie it was marvelous, and it inspired me to write the song.