From p88 of Cnósach Focal ó Bhaile Bhúirne:

Diongbhálta should mean "steadfast, worthy", but "uiriste rubairt air" (where rubairt is a form of forbairt) means "someone who is easily put out/offended"? This seems to be the opposite of the normal meaning.

Since it's "Is uiriste rubairt air" rather than "Is uiriste rubairt a chur air" it means "he bears irritation easily", i.e. literally "Irritation is an easy thing on him".

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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

Well, this is glossed in Ó Dónaill's dictionary: https://www.teanglann.ie/en/fgb/forbair

Is furasta forbairt air, he is easily put out

[I suspect a lot of entries in Ó Dónaill are taken from Cnósach Focal ó Bhaile Bhúirne and slightly standardised. I've seen a lot of exactly matching phrases.]

I've only ever heard "diongbhálta" to be equivalent to the English "unshakeable", for example in common phrases like "creideamh diongbhálta" or "creidim go diongbhálta". In the early 19th century poem "Searcleanúin Chríost" the phrase "duine diongbhálta" occurs with the meaning of an absolutely steadfast person. Similarly I would have read that sentence as being about bearing irritation easily. I'll have to look into it more.

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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 asked an older native speaker (in his 80s) and he read:

"Is uiriste rubairt air"

as "he's not easily put out" literally "Irritation is easy on him".

Not sure how to explain this given what Ó Dónaill says.

If it's interesting the speaker used forbairt.

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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

A Londubh, Dinneen also has this phrase (under forbairt), glossed as "he is easily put about", but I'm not aware of "put about" as a phrase. Is this common in Irish English? Or is this older English? Does "put about" mean "put out"? Maybe this phrase is rare or odd -- a lot of sentences in CFBB seem a bit odd (as they are all completely devoid of context).

Interesting. All I can add is that most native speakers I know have "forbairt" as a word learned at school from Standard Irish, they wouldn't naturally use it. As far as I have read rubairt was the Munster form. However even older speakers don't say it that way today. Most wouldn't be aware that forbairt even had the meaning of annoyance. So probably the sense of this phrase has been lost.

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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

Are you saying that this word just isn't used in speech then?

Forbairt to mean "development" does not have a long history to it. Before the 20th century, almost every instance of the cognate verb attested is in the phrase "d'fhás agus d'fhorbair", "grew and increased". It has been adapted in the modern period to mean "to develop" with the verbal noun meaning "development". And yes, to the extent that the word is heard in the media, people will know what it means, but it is not a Gaeltacht word as such (other than the fact that all CO words will have at least some recognition in the Gaeltacht from those exposed to the media). Looking on Youtube, there is "Forbairt Bhóthair N22, Bhaile Bhuirne go Maigh Chromtha". It is to a certain extent natural that new words are required in all languages in the modern period (but many of them are mainly used by L2 speaker). To develop the language = an teanga a shaothrú. N22 road project could be tiúscadal Bhóthar N22.

Just out of curiosity, how did you come across this poem? I see that it's on the RIA corpus but I can't find any other mention of it on the internet. Sometimes I like to browse through their collection of texts to find new things so I assume you did the same but are there poetry collections that would be good to read through?
Thanks