Men are capable of singing two different ways. In the first way, only the edges of the vocal folds come together. This is how children and women sing. At puberty, a male's voice starts to change, and he comes to sing (or speak) with more than the edges coming together. His voice sounds deeper.

However, some men keep the knack of the childhood style and can sing very high and pure. This style is called "countertenor." I once went to a concert by a countertenor, who said that in the old days, it was quite common for a man to sing one part of a song in his adult style and one in countertenor. He sang "Molly Malone" that way to demonstrate.

I have thought since then that "Danny Boy" would be a logical song to receive this treatment.