It isn't a Cape Breton song, though it has been recorded by Cape Breton performers (among others). Probably your difficulty in finding it is down to variations in spelling; 'Seallaibh Curaidh Eoghain', for example, will return mostly references to the song as performed by Sileas; while 'Seallaibh Curaidh Eoghainn' is how the Barra MacNeils spelled it. 'Seallaibh curaigh Eoghainn' gets better results; not in terms of quantity but in that it will locate a text, with translation, at  http://www.geocities.com/area51/zone/6338/puirt7.html. George Seto's website, as it turns out.

It is credited there as 'from the singing of Mrs Annie Arnott, Benbecula'. Although further detail isn't given (and Mrs Arnott didn't belong to Benbecula but to Skye), the lyric was presumably copied from the sleevenotes of Scottish Tradition vol 2: Music from the Western Isles (School of Scottish Studies/Tangent Records TNGM 110, 1971; since re-released on CD by Greentrax: CDTRAX9002, 1992). The record was quite influential and became a source of material for a fair few modern performers; the Bothy Band got 'Fionnghuala' from it, for instance, with the result that a lot of people now think that's an Irish song.

No information is given in the sleevenotes about Mrs Arnott or the recording date, but the following is from Tocher (Edinburgh: School of Scottish Studies, I, 1971, 5):

'Mrs Annie Arnott is a native of Kilmuir, Skye, where she has come back to live after many years in Glasgow. A rich tradition of Gaelic song has survived in Kilmuir until the present day, and Mrs Arnott is one of its foremost exponents. She was first recorded in 1950 by Derick Thomson (now Professor of Celtic in the University of Glasgow) and subsequently by the late Calum MacLean and other members of the School of Scottish Studies.'

Four of her songs (though not this one) are transcribed. One of those, 'Iomaraibh Eutrom', together with a further two, can be heard on Hamish Henderson Collects, Volume 2: Songs, ballads and a story from the School of Scottish Studies Archives (Kyloe Records 110, 2006).