Fascinating stuff.
According to the Porter and Gower book, what Jeannie Robertson sang was "For I took out my gun, and my dog I did show't All down by the River Clare." I shall not attempt to explain "show't". In the book is an interesting consideration of the Irish influences on [rather than Irish origin of] the tune and chorus text. There I think no Scottish River Clare. Is there one in Ireland?
On the other hand, in Sheila Stewart's own new booklet she says it was written about a relative of hers. I only skimmed the booklet yesterday, but I do not think she states who she believes wrote it.
By the way, as Porter and Gower observe, there is another totally dissimilar song of the same title in the Greig Duncan collection.