I am trying to learn some Irish on Duolingo. In addition to Duolingo, I have been doing some other research / studying, because Duolingo does not explain some things. So, I was putting together a study sheet, and although I am unlikely to ever use the phrase, I wanted to add "Where are you from". Duolingo has "Cad as tú?", but Google Translate and Wiktionary both have "Cárb as duit?" and "Cad as duit?" and neither has "Cad as tú?"

Which one is right?

They're all correct.

Like in English, there are many ways to ask the same question:

Where are you from?
Where do you come from?

or colloquially

So, where do you call home?
Where is home to you?

Thank you!!

There are different words for "where ... from"/"whence" in Irish:
cad as "what out-of-it" ("as" is a combination of as = out of and é = it)
cárb as "where out-of-it"
and some more: cérb as, cé as, cá has, …

The "verb" of such sentences is "is" (the copula) which is understood here.

The subject can be simply "tú" or "thú" ("you") but in sentences/questions with "is" the subject is sometimes combined with the preposition "do" ("to, for"), so: "duit" (lit. "to-you")

If you combine all of this you will have a lot of versions, used depending on dialect and personal preferences:
Cad as tú/duit?, Cárb as tú/thú/duit?, Cérb as thú/duit?, Cé as tú/thú/duit?, Cá has tú/thú/duit?, …

Do replies to the question as to where someone is from also have to use the copula? I had thought so but then I came across this sentence that used the substantive verb: "dúirt Lola níos déanaí go raibh sí as Luimneach ó dhúchas." at https://ga.wikipedia.org/wiki/Lola_Mont ... hs%C3%B3ir)

Well, let me say that I regard that as just wrong.....

Yes.
"bí + as" is confined to phrases like: as mo mheabhair, as mo chéill, as radharc, as baile, as láthair, and a few more. It means "being outside of"

Duairt sí gurb as Luimne ó dhúchas di.

Actually, I think you can swap the "as" for "ó". Both are found, I think.

Labhrás, Djwebb, thanks for the replies.

'Cárb as tú?' (Where are you from?) seems more logical to say than 'Cad as tú?' (literally 'What are you from?'). The latter does seem a little peculiar when you think of it literally.

Now that I think about it, a sentence like 'Is as Luimneach dó' seems peculiar also since a prepositional pronoun is acting as the subject of the sentence, which doesn't seem very rational. I wonder how this came about.

Also, if the following equation holds: 'Is as Luimneach é' = 'Is as Luimneach dó', do the following equations also hold:

'Is as Luimneach Seán' = 'Is as Luimneach do Sheán'
'Is as Luimneach an fear' = 'Is as Luimneach don fhear'

(I don't think I've come across 'do' plus noun to express where someone if from.)

It was already so in Old Irish.
The copula "is" doesn't meant "is", actually.
It rather meant "he/she/it is". So, there was already a subject. "Is" was a combination of the copula and a 3rd person, like "tuigim" is a combination of tuig and first person. ("Am" meant "I am", "at" meant "you are").

And the thing/person involved was added as a mere apposition
- either in nominative (so in sentences with a noun as predicate)
- or as an apposition with do (so in sentences with an adverb as predicate).
The phrase "as Luimneach" is adverbial, so you can use "do".

"Is as Luimneach do Sheán" meant literally (or rather "etymologically") "He's out-of Limerick, (for) Seán"
and "Is fear (é) Seán" meant "He's a man, (he) Seán"
In both cases "Seán" was not really the "subject" but only an apposition to an already complete sentence.

I don't know for Old Irish but in Modern Irish this "do" is a relict and so it is optional.
But there are a few phrases where it is still mandatory, e.g. "Is ann do Dhia", God exists. "Is ann é Dia" would be wrong.



Yes.
See also: "Is from" question here in this forum.