Ha! That's mad, aerach is a Munster dialectal spelling of the verbal noun for "herding/sheparding", so it made sense that way too.

If that's the context then yes:

It's a great worry the cows were to the fairy host

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

I'm still stuck on this one. I don't understand the meaning and usage of - na ba - at the end of this sentence.

Ba mhór an ceasadh don tsluaigh aerach na ba.

If anyone could help please, I'd be most grateful.

"Na ba" refers to the cows, it's a copular sentence:

Ba mhór = It was great

an ceasadh don tsluaigh aerach = the worry to the fairy host

na ba = the cows

=> The cows were a great worry to the fairy host.

"Na ba" = "the cows" is the subject of the sentence. The rest of the sentence is information about them (i.e. that they were a great worry to the fairy host)

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Ah OK - I thought it was a repetition of the verb. Go raibh maith agat.

Here are two more expressions I'm stuck on. Here's the context -

Ba dhoiligh leo géimneach na mbó ar na plásóga.

Ba dheallrathach é le caoineadh na marbh, dar leo, agus fé dheire an bhainríoghan - bainríoghan na sidhe - teicheadh agus fán do chur as an áit ar an lucht creachta, idir bólacht agus aodhaire.

I'm guessing that it means she going to flee and leave the place to the plundering people - but I'm not sure about the expression - fán do chur as an áit - and does - idir - as used here, mean both or between?

Oidhche áirithe nuair a bhí gealach an fhóghmhair ag taithneamh go mamhrach anuas ar an gnoc....
- I can't find - go mamhrach - anywhere in the dictionaries including Dinneen.

Tá iontas orm nár tugadh freagair ar do cheist go foill...

1 - No idea about go mamhrach.
2 - In this context idir = 'both', though in the CO it lenites.
3 - fán do chur ( in modern Irish do >a. Under fán FGB has Is tú a chuir fán orthu, translated as "It's you who sent them adrift" (not in the literal sense, and - out of context - not very helpful).

I googled around and, from the few examples I found, fán a chur (ar dhuine) seems to mean something like "to drive away/disperse/banish/exile". I think teicheadh is also covered by do chur: ( usually with the article) an teitheadh a chur ar dhuine - "to put to flight/drive away/rout". However, I'm having a wee bit of trouble with the bit starting from fé dheire an bhainríoghan. Have you copied the text accurately, franc? It seems to me there's a verb missing before an bhainríoghan:

"...at last the queen (missing verb - decided/tried/gave the order?) to put the raiders to flight and drive them from the place ". Not sure where the coos and herdsman fit in.

On the other hand it's just as likely it's me that's missing something.

Go raibh maith agat a chara.

I can give you the original version in English on which she has based her Irish version.

In Tipperary is one of the most singularly shaped hills in the world. It has got a peak at the top like a conical nightcap thrown carelessly over your head as you awake in the morning. On the very point is built a sort of lodge, where in the summer the lady who built it and her friends used to go on parties of pleasure; but that was long after the days of the fairies, and it is, I believe , now deserted.
But before the lodge was built, or acre sown, there was close to the head of this hill a large pasturage, where a herdsman spent his days and nights among the herd. The spot had been an old fairy ground, and the good people were angry that the scene of their light and airy gambols should be trampled by the rude hoofs of bulls and cows. The lowing of the cattle sounded sad in their ears, and the chief of the fairies of the hill determined in person to drive away the new comers; and the way she thought of it was this. When the harvest nights came on, and the moon shone bright and brilliant over the hill..........

.... and the cattle were lying down hushed and quiet, and the herdsman, wrapt in his mantle, was musing with his heart gladdened by the glorious company of the stars twinkling above him, she would come and dance before him, - now in one shape - now in another, - but all ugly and frightful to behold.

Agus seo an cleas a d'imir sí ortha. Oidhche áirithe nuair a bhí gealach an fhóghmhair ag taithneamh go mamhrach anuas ar an gcnoc agus na ba ag cosaint na cíorach ar a suaimhneas tar éis an lae, agus an t-aodhaire ina shuidhe go seascair ag machtnamh dó féin insan uaigneas, tháinig an bhainríoghan ag damhsa ós cómhair a súl. Ach ná ceap gur ina riocht féin a tháinig sí, mar níorbh' eadh in aon chor.

- I've checked the text in Irish and there's no extra verb - it seems to be assumed in the meaning. As you can see, she has rewritten the story in her own idiom, rather than translate it.

I'd forgotten it's an "interpretation" in Irish of the English original. Still, I don't see how the "missing" verb can just be understood,"assumed" . It really has to be there, I think. Anyway, I see from the ainm.ie website that the Brothers Grimm translated the original into German.

Ah you're right ! agus fé dheire do bheartuigh an bhainríoghan - ....(does that make more sense ?)