The Christy version of this song seems to look upon the girl more sympathetically - perhaps the green lily referred to is the her. Green because she is young, naive, not fully matured, innocent etc. If she is compared to a lily, maybe it is to plant the idea that she is beautiful (also slightly sinister as the lily is synonymous with death, I think?)
I'm very curious to know what people think of the "my cup is full up to the brim, if I were to stoop I might fall in" line?
All the natural symbolism ties in well with the bodhrán - very pagan and earthy ('she swore by grass, she swore by corn"). I love this song - although I skip over it if I'm listening to it in bed as it frightens me a bit!