I think we are here again at the chicken and egg stage without even being able to decide whether the English or Irish verses are respectively chicken or egg. I expect that there were words in Irish to the tune before Campbell composed his verses to it. Whether those words are the same as the Irish version given on this site is another question and would require specific research. Does anyone know for instance the earliest source of the Irish language text?

As a parallel, we might think of the Seoithín Seo lullaby discussed under a different thread here - also given under the title Mullach an tSí to avoid confusion with other Seoithíní. The commonly used Irish text to that was written by Tomás Ó Ceallaigh (and subsequently translated into English). However there were words to the tune before that (possibly changing every time the song was sung) and two versions are given in Amhráin Mhaighe Seola. What Pádraigín Ní Uallacháin seems to have done (although this is not stated in her notes and there might be an independent source) is to put the uiseo, uiseo vocables from one version into what was Tomás Ó Ceallaigh's verses. I'd imagine the Uiseo, uiseo version (given to Mrs Costello by Micheál Breathnach) is a witness to the way in which the lullaby was sung before Ó Ceallaigh supplied his verses.