I'd uncovered the willow reference, but couldn't quite make it fit with the usage I saw in google. That site has a pretty good explanation. It mentions a poem by Yeats, as well as the text of a song about "Sally's garden", both excellent pieces.
Interesting, arnie. Your site says, "The primary modern meaning for Sally port ... is a small controlled space with two doors. Essentially, one must enter the space and close the first door before opening the second to proceed."
But there's no such meaning given in the usual online dictionaries. No reference to a pair of doors. For instance, MW says "a gate or passage in a fortified place for use by troops making a sortie", and AHD is much the same.
Sean, on your original question I haven't a clue. In what context did you find the term?
Maybe this relates to the original question....There's a common plant (I hesitate to call it a weed since, as kids, we used to eat it)that grows wild by the roadsides and in untended gardens, and we always called it "sallies". I'm not sure, but maybe it's also known as Lamb's Quarters.