Leora the song is about.
The "whole of the moon" is the big picture, which takes a particular vision to see. In a Songfacts interview with Waterboys frontman Mike Scott, he said: "I wrote the song when I was 26 years old, and I was discovering that there was so much more than I had ever known. There was so much more to learn than I'd even been hinted at in the culture I'd grown up in. I had a strong sense of wonderment about that, and I realized there were people who had vastly more information in their imaginations and experiences than I had. And so that's what inspired that song."
Original song, you can hear the words better.