Bill Kennedy suggests seoithín is from seod.

The d and t sounds are similar in Irish (more so than th & dh, but those are both soft sounds. You might have something there, Bill, if you sang "this little gem" you'd be drawing the fairies the attention to the fact that you have jewels, and human jewels, at that in your company. That's what I was told about "Bog braon don seanduine", in any case, that the baby is called an old man in order to fool the fairies, who are prone to stealing babies.

I suppose you've seen some of the several threads discussing the function and meaning of "musha ring dum a day", "whack fol the diddle", etc. Some of the messages contain suggestions that these choruses are attempted transliterations of Gaelic, or even of Pictish or Druidic tongues. One response to the mangled Gaelic theory is that several of the Gaelic language songs themselves contain vocables.

As it so happens, I've typed out several lyrics today that contain untranslatable refrains - Seoithín Seo, Port Láirge, Dónall Ó Conaill, Láirín Ó Lúrtha, Fuair Mise Cuireadh, 'S ambo éara, Deoindí. In some cases, certainly the last two songs mentioned, the refrains give a frame work to improvise lines around. In some of the songs I've mentioned the refrains are similar to the ones we get in English-language songs from Ireland, "Raics dí al" (Rex dee al), "raight [right] fol do dol dol". These songs are not that ancient, so their singers would have been familiar with English. [thread is creeping - I plan to copy this section of the discussion and add it to one of the aforementioned threads about nonsense lines]. I wonder if one of our historical or linguistic experts can come along and tell us whether the refrains of Port Láirge, Fuair Mise Cuireadh and Dónall Ó Conaill originate from Irish or English language traditions. Annraoí might have to broaden his definition of macaronic song!

The most amusing scenario would be that the Right-fol-the-dols in English-language songs were indeed mangled transliterations of real Gaelic words and then got transliterated back in a new generation of Irish-language songs. The vocables in Scottish Gaelic songs have a consistency about them and often their importance in keeping rhythm can be clearly seen; sometimes they also serve to identify the tune to be sung when the words vary (a bit like the function of the refrains in 'S ambo éara and Deoindi). When the song A' Bhean Eudach travelled from Scotland to Ireland, sometimes the Irish Gaels who were unfamiliar with the Scots refrains such as "huir í ó bhó" tried to make some sort of sense of them with lines such as "a shiúir i gceo"