There is more discussion on earlier threads. Here is some info. from 1999 contribution:

"The following information ... is from Ben Buxton. "Mingulay: an Island and Its People". Birlinn, Edinburgh, 1995

p. 47-48

"'Mingulay's most famous song - outside Barra and Vatersay that is - is "The Mingulay Boat Song'. But neither the words nor the melody originate anywhere near Mingulay; it is a romantic invention of the 20th century. It was devised in 1938 by Glasgow-born Sir Hugh Roberton, who was very fond of the melody of 'Creag Ghuanach', a song from Lochaber, which celebrates a crag near Loch Treig. He needed a sea shanty, and so he adapted the music, chose the romantic name Mingulay, and composed the words. It was to be sung in F, slowly and rhythmically. [Roberton Publications, personal information; Derek Cooper. "The Road to Mingulay: a View of he Western Isles", London, 1985]…

"'It is ironic that this song should be the only well-known song associated with the island, and, for many, the only reason they have heard the name Mingulay at all.'

"In his appendix, Buxton publishes two real Mingulay songs, in Gaelic with Englsh translations: 'Oran do Dh'Eilean Mhiulaidh' /'Song to the Isle of Mingulay' and 'Turas Nèll a Mhiùghlaigh'/ 'Neil's Trip to Mingulay'"

Nice to see you on Mudcat again, Cuilionn! While you're right that the tune (not the words) of the Mingulay Boat Song carried Gaelic words, the song you mention was composed by an Irishman - who is still alive - who set it to the air he knew from hearing the Mingualay Boat Song. The modern Irish Gaelic song was subsequently translated to Scottish Gaelic. See An Cailin Alainn