The Gaelic language song is a translation of "The Lambs in the Green Hills" https://mudcat.org/@displaysong.cfm?SongID=3491 and is sung to the same tune as that usually used for "The Lambs in the Green Hills". I think of The Lambs in the Green Hills as an Irish song*. The best known Scottish version of this old song is "I Aince Loved a Lass" (or I Once Loved a Lass).

*The "Mainly Norfolk" website concurs; The Lambs in the Green Hills was in the repetoire of source singer Sarah Makem of County Armagh as well as having been published in Colm Ó Lochlainn's Irish Street Ballads. There is an amusing misprint of Séamus Ennis' surname, but I don't find "Seamus Ennui" boring! https://mainlynorfolk.info/copperfamily/songs/thefalsebride.html

The translation of The Lambs in the Green Hills to "Tha na h-Uain air an Tulaich" is credited to Seonaidh Ailig Mac a' Phearsain aka John Alick MacPherson (1937-2017), a native Gaelic speaker.
https://www.bbc.co.uk/alba/oran/orain/tha_na_h_uain_air_an_tulaich/

Seonaidh Ailig Mac a' Phearsain air bàsachadh - Naidheachdan a' BhBC

Scotsman newspaper obit and potted biography of Seonaidh Ailig