The 'sally' or 'salley' that the song refers to come from one of the celtic languages word for 'willow', which is 'saille'. (And it's pronounced like 'sally'.)
I gave that book back to my friend, so I don't know which celtic language it is.
The latin name for willow is 'salix'. That is why Aspirin is 'salicylic' acid.
Aside from that, Dave Oesterreich is right.