I remember reading somewhere that 'Buachaill' simply means 'youth.' At one time, cattle-raising was so central to the Irish economy that the term cowherd was generalized to mean any young male.

In my version of O'Neill's Music of Ireland, 'Buachaill' is translated as 'boy' in these songs

The boys in the gap
The black slender boy
I'm a poor rambling boy.
The brown-haired boy.

No doubt there are others.