This has a better ring to it.

Hmmm... my gut tells me even if it is a normal copula with a predicative adjective it shouldn't have é
Like wouldn't it be "Is maith an rud a dúirt sé"?
Honestly I cannot think of any possible way to say a copula sentence with a predicate adjective that involes in indefinite noun instead of a definite one. All of the examples that my mind creates use genitives or relative clauses....
Is it possible to use one with an indefinite noun? And is é really allowed?

grma

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Is maith an rud ((é) an rud)* a dúirt sé = lit. It is *a* good thing (the thing that/) what he said.

This is a different construction (though it seems similar)
It is like #3b in the following list.

* (BTW: an rud doesn't occur twice, the second an rud (= the subject) is understood. That's why I put it in parenthesis. Again there would be the choice to use a dummy sub-subject pronoun é, see the example with a different subject in #3b.

Compare:
#1 Is maith an rud. = The thing is good. (an rud -> subject, maith -> predicate)
#2 Is maith é an rud. = The thing is good. (é -> sub-subject, an rud -> subject, maith -> predicate)
#3a Is maith an rud é. = It is a good thing. (é -> subject, maith an rud -> predicate)
#3b Is maith an rud (é) an t-alt a scríobh sé. = The article he wrote is a good thing.




Is geal solas. = Light (in general) is bright.
Is geal an solas. = The light (in general) is bright.
There's not really a difference between definite/indefinite here. Both are meant to be generalising.
(Because of this generalising tendency "Is maith rud" "thing (in general) is good" doesn't make much sense.)

But compare with:
Is geal é an solas = The (special) light is (astonishing) bright. (no generalising at all, other lights aren't that bright)

(and de réir #3a above: Is geal an solas é. = It is a bright light.
or de réir #3b Is geal an solas (í) an ghrian. = The sun is a bright light.)

I completely follow all of these, like 100%.
BUT ... is it then impossible to say Is geal é solas? Because that is what I am asking about. It seems to me that é cannot be used with indefinite nouns. Perhaps I was not entirely clear, but the things you have (so obligingly, clearly, and helpfully) explained are things which I do understand as opposed to the idea of é with something indefinite of whose permissibility I remain in doubt

EDIT: Actually now that I look again I was also asking about the "is geal solas" so thank you

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É doesn't make sense here.
But I would never utter the word "impossible" in association with Irish language

touché

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