My comment 3 years ago was based a remark made by Bertrand Bronson in his essay, Samuel Hall's Family Tree (California Folklore Quarterly I, 1, 1942), reprinted in The Ballad As Song (University of California, 1969). Note that the date I mentioned was for the earliest published example that I had seen at that time, not for the original publication; which as Bruce pointed out was much earlier (1794).

Although authorship was claimed by Hector MacNiell (according to William Stenhouse), this has been disputed. Bronson adds -without comment- that Orkney tradition ascribes Germany Thomas to a Colonel Traill round about 1625; but that is anecdotal. It would be that claim, I expect, that Bonner quoted from Miss Gilchrist (did he give a reference? I don't have time just now to plough through the Journals of the Folk Song Society again). The earliest known Orkney example was printed in 1885, it would appear (Colonel D Balfour, Ancient Orkney Melodies).

Some material from Balfour is quoted at http://www.electricscotland.com/history/germany/germany_thomas.htm

Bear in mind, of course, that the story is anecdotal and is not necessarily true; local family tradition also, it seems, had it that Traill had written Ten Thousand Miles. Whether written in the form under discussion here by MacNiell or Traill (or neither of them) My Love's in Germanie / Germany Thomas was certainly based on an older piece; Bronson quotes a reference in The Complaynt of Scotland (c.1549) to a song beginning

My lufe is lyand seik, send hym ioy, send him ioy

but that line is all we have of it.