Mark,
Thanks for the words to the Willow Garden. I wasn't familiar with it and it obviously has no relation to Yeats' Salley Garden except that it is set in a willow garden, has strikingly similar first two lines and is in basically the same meter.
I am confident about sally or salley meaning willow although I can't say where I learned it. further tomorrow. I do know that the scientific name for the willow genus is salix, and that might have some bearing.
David