I've now searched, with the help of the Irish Traditional Music Archive, for references to either "Carrickfergus" or to "Do Bhí Bean Uasal" (They turned up 154 references to recordings of Carrickfergus - confusion is easy.)

In the first case - nothing appears prior to 1961. Concerning "Do bhí bean uasal" there are two significant references - an Irish language newspaper of 1905 (Iris Leabhar na Gaedhlige - spelling not guaranteed) and Donal O'Sullivan with Mícheál Ó Súilleabháin (eds) Bunting's Ancient Music of Ireland (Cork, 1983) (no 72). The notations edited date from the late 18th to early 19th century.

In neither case is the tune given the same as, nor does it bear any but occasional resemblance to, the one well known as "Carrickfergus."

The version of "Do bhí bean uasal" remembered by Neil and Martin was sung by Seán Ó Sé on a record "O'Riada sa Gaiety" (Gael Linn CEF 027) which selected a concert given by O'Riada in late 1969. The performance intermingled some of the Irish words of "Do bhí bean uasal" with some of the English words of "Carrickfergus" and used the air of Carrickfergus. These two sets of words (if those in Bunting are a guide) are not related.

Given the differences between the tunes of "Carrickfergus" and "Do bhí bean uasal" and the lack of connection between the texts, I am of the opinion (unless other evidence emerges) that their juxtaposition by Seán Ó Sé was no more than opportunism - two songs for the price of one and to a tune everyone knew.