How would you say "we saw that we were/had been/are pawns"?

any of the above variations would be acceptable...I'm just not sure how to use the copula when it's subordinate like that.

I've been suggested, "thuigeamar gur cheithearnaigh sinn"

I think this is a case where the meaning lies elsewhere. The use of the word "pawns" would not be customary in Irish but there are other ways of saying "we had been duped / deceived / misled etc" which I think may be the underlying meaning. If so say "Cuireadh dallamullóg orainn" or "Buaileadh bob orainn" or "Mealladh sinn".

If you want to home in on the idea of us being sacrificed "rineadh íobartaigh dinn" or abused "rinneadh íospartaigh dinn" and so on. Look up "íobair" and "íospair".

There is always a clear alternative in Irish to the English phrase. If the English phrase is translated literally it will be very obvious to all that it is a translation.

Regarding the word "pawn" as used in chess either "ceithearnach" or "fichillín" could be used.

The most immediate meaning I would take from "ceithearnach" would be "an outlaw" "someone hiding out in a forest" etc. Unless mentioned in the context of chess I would never associate it with "pawn" although it is mentioned in Ó Dónaill as one of about five meanings.

Regarding the copula: take a different sentence: "Is comrádaithe sinn" (we are comrades) or in the future "is comrádaithe sinn feasta. (i.e. from now on). In the past that would be "ba chomrádaithe sinn" we were comrades [but are not now].

If you want to put an introductory verb before those and thus put them into "reported speech" you should know that the tense of the introductory verb affects the tenses used in reported speech as in English: We are comrades: He says we are comrades; but "he said we were comrades" and if it were "we will be comrades" -- he says we will be comrades; and "he said we would be comrades". So to understand the changes you will need to look up your "Claoninsint" or "Reported Speech" in your grammar and consider the changes wrought by the "Aimsirí Príomha" and "Aimsirí Stairiúla" in the "Briathar Réamhrá".

Is comrádaithe sinn (present tense)
Deir sé gur comrádaithe sinn ( ... nach comrádaithe ...)
Dúirt sé gur chomrádaithe sinn ( ... nár chomradaithe ..)

Ba chomrádaithe sinn (past tense or modh coinníollach)
Deir sé gur chomrádaithe sinn (... nár chomrádaithe ...)
Dúirt sé gur chomrádaithe sinn (... nár chomrádaithe ...)

Is comrádaithe sinn feasta (future tense)
Deir sé gur comrádaithe sinn feasta ( .. nach comrádaithe ...)
Dúirt sé gur chomrádaithe sinn feasta (...nár chomrádaithe sinn ) (= modh coinníollach)

That's the best I can offer. I hope it is correct.