Richie, in this case I think we can rely on the copyright date.

Roberton's "Songs of the Isles" (1950), p. 36, includes the Uist song with the 1937 copyright notice.

The credits read, "The original Gaelic words by Archibald MacDonald, Uist. The English words by Hugh S. Roberton. Tune by John R. Bannerman arranged by Hugh S. Roberton."

It thus seems that the "song" originated as a Gaelic poem by MacDonald, whose words may or may not have been translated by Roberton himself.

Roberton identifies most of the melodies in "Songs of the Isles" as "Traditional" or "Old Highland air" or similar. But a few are clearly attributed to current composers as original tunes.

"Westering Home" (crt. 1939 by Hugh S. Roberton) is interesting because Roberton claims the words and the stanza melody, but attributes that of the chorus to "the singing of Donald McIsaac."

Roberton was evidently scrupulous in distinguishing the traditional from the brand-new.

McIsaac's tune is easily recognizable as a version of "The Muckin' o' Geordie's Byre."