I've recently read 'Angela's Ashes' a autobiography or memoir written by Frank McCourt and Irish author. Within said book is a sex scene in which Frank takes off all his clothes and gets and erection, his partner looks at him and says "Lord, you might be a scrawny bit of a fellow but that's a fine boyo
you have there". This leads me to believe that in the 1930's in Ireland, more specifically Limerick Boyo
was a slang term for a penis.