I heard Frank Harte sing this in Goilin many years ago. He introduced it by saying it was a song about impotence.   The age difference emphasised in the first lines, and the young woman’s choice to go with the “gruaigaire”, young man in both original Irish and current English versions has always seemed to me to legitimately suggest this.    She obviously does think it is “fair” to go off with the young man! Why?   
The contrast with the Tory island ending in Irish, which is quite cheery, is interesting. Stephen’s as a translator had both his limitations and his brilliant lines. I wonder did he think a life with the Brown and Yellow ale was contributing to the old man’s limitations, and it added to a mis-translation.