The first message in this thread references The Speckled Book, which is equally well-known to scholars as the Leabhar Breac, and is preserved in Ireland. This manuscript is the source of more than one poem, hymn, or commentary from the margins, in Old Irish, which found favor with Barber and other composers.

Besides "Jesukin," which is a literal translation of the Old Irish "Ísucán," there is another quatrain which I would like to post to this thread: also from the margins of the Leabhar Breac. This time the scholar in question was not Whitley Stokes, but the German-born Kuno Meyer.

The quatrain was first published in the Irisleabhar na Gaedhilge / The Gaelic Journal, dated September 1, 1894, Volume V, no. 6, on page 94. The Old Irish complicates the spelling of what is actually a heart-breakingly simple text.

Do gabsat ó gaírm in chét eoin
'cot chrochad, a ghrúd mar géis:
nís chóir anad oc cói chaidche --
scarad lái is aidche da éis.

Kuno Meyer published more than one translation of this quatrain. Here is the earlier translation which appears in the Gaelic Journal along with the original text.

At the cry of the first bird they began
To crucify thee, O cheek like the swan:
It were not right to cease lamenting ever --
Parting of day and night after it.

By the time Samuel Barber got hold of this striking bit of poetry, it had turned up credited to Howard Mumford Jones (credit, that is, for the translation) in his 1000 Years of Irish Poetry and The Romanesque Lyric. Jones, in fact, did little to improve upon the translation made in the first place by Kuno Meyer. Here is the version Meyer published later:

At the cry of the first bird
They began to crucify thee, O cheek like a swan!
It were not right ever to cease lamenting --
It was like the parting of day from night.

Ah! though sore the suffering
Put upon the body of Mary's Son,
Sorer to Him was the grief
That was upon her for His sake.

-from Selections from Ancient Irish Poetry, 1911.
These two quatrains have no attributed author. In fact, they appear in two different spots in the Leabhar Breac / Speckled Book, in the margins of the manuscript. At some point Kuno Meyer decided to put the two quatrains together. They have remained together from that day to this. I cannot discover if Meyer, or anyone else, has ever published or printed the original Gaelic / Old Irish version of "Ah! though sore the suffering."