It seems to be a translation issue, coupled with people's subjective impressions of what they think they have heard on commercial recordings.

Kennedy-Fraser printed it untranslated as 'Ben a Hayich', which was intended to indicate an approximate pronounciation to the non-Gaelic speaker. Usually given as 'Beinn a' Cheathaich' and translated as 'The Misty Mountain'. Flora MacNeil of Barra, the source of the best-known 'real' Gaelic version, seems to have concurred in this.

Our 'Guest' seems to have added his or her 'correction' to several old discussions of this song. My impression is that he or she has simply misunderstood somebody's recorded arrangement of the Kennedy-Fraser 'drawing-room' version of the song, as did those who thought it was 'Bennachie', 'barenock heeya' and other such.

For versions noted from tradition, see:

John Lorne Campbell and Francis Collinson, Hebridean Folksongs: A Collection of Waulking Songs Made by Donald MacCormick in Kilphedir in South Uist in the year 1893 (Oxford University Press, 1969, 150-153 [text] and 335-337 [tune; two versions from Barra]).

John Lorne Campbell, Songs Remembered in Exile (Birlinn, second edition, 1999, 128-132: text and two tunes noted in Nova Scotia, 1937).

Peter Kennedy, Folksongs of Britain and Ireland (second edition, Oak Publications, 1984, 30: from Flora MacNeil, Barra, n.d.)

These all render 'Beinn a' Cheathaich' as 'Misty Mountain'.