John, thanks for refreshing this. I'd hoped somebody else who knows more on the subject might help. I can't find any reference to the Wexford Carol(s) ever having been in the native Irish language, but from the information I've seen it seems likely that the version Brían heard is a modern translation. Possibly a request for information to Raidió na Gaeltachta might help out here.
In the meantime here is some information about the Wexford Carol(s) as there doesn't seem be anything in the DT or Forum.
This page gives a list of publications about the Wexford Carols, while this book Folklore of County Wexford edited by Drs Diarmaid Ó Muirithe and Deirdre Nuttall, 1999 also contains some information about the carols.
The particular song generally known as "The Wexford Carol" is the one beginning "Good people all, this Christmas time" from The Oxford Book of Carols (No. 14). The editors' notes state that the carol was "kindly communicated...by Dr. Grattan Flood. The words (subsequently revised) and tune were taken down from a traditional singer in County Wexford. The words seem to have come from England..."
This carol belongs to a group of carols still sung in the village of Kilmore, Wexford to this day and some useful information is given in The Companion to Irish Traditional Music ed. Fintan Vallely, Cork University Press, 1999. The article (summarised below) is based on Diarmaid Ó Muirithe's book, The Wexford Carol.
The original set of eleven Christmas songs in A Smale Garland of Pious and Godly Songs was published in Ghent in 1684 by Luke Wadding, the Catholic Bishop of Ferns, and a member of one of the principal Anglo-Norman families of Wexford. These songs were the foundation of a tradition of carol-singing in the county of Wexford.
After Wadding's death the tradition was given new impetus by the manuscript collection of carols made by the parish priest of Drinagh, Fr William Devereux, in 1728. It is this collection, A New Garland Containing Songs for Christmas, which is the basis of the continuing tradition in the village of Kilmore where the choir still maintains the custom 'that there must be a Devereux among the carol singers'.