Here is the first part of Isaiah 41:17 from Bíobla Naofa:

Tá na dearóile agus na bochtáin ag lorg usice agus níl sé ann. Tá an teanga spallta iontu le tart.

I translate this as:
"The puny and poor are looking for water and there is none. The tongue is shriveled in them with thirst."

Is this correct?

Can somebody help me fix this translation up so it expresses the following:

"When the puny and poor are looking for what and there is none. When their tongue is shriveled with thirst"

So basically I want it to start with "when".. I'm wondering how much of the sentence structure would need to change.
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Nuair atá ...

Or nuair a bhíonn…, depending on the actual idea expressed.

Is this a correct translation?

"When the insignificant and the poor are looking for water and there is none. When their tongue is parched with thirst."

"Nuair a bhíonn na dearóile agus na bochtáin ag lorg usice agus níl sé ann. Nuair a teanga spallta iontu le tart."

No.

Nuair atá na dearóile agus na bochtáin ag lorg usice agus níl sé ann. Nuair atá a dteanga spallta iontu le tart.

I’m inclined to use atá here as tá was used in the original. Nuair a bhíonn isn’t wrong, but if the original were Bíonn na dearóile agus na bochtáin ag lorg usice… I would be more inclined to use it.

The use of “tongue” (singular) also seems odd, but again, as that’s what’s there, that’s what I’d translate.

Tongue in singular is normal Irish usage. When everybody has one -> singular.
ár gcroí, ár dteanga (not: ár gcroíthe, ár dteangacha)

I could have been clearer about that. I meant that it seems strange for it to be singular in the English, but, it being the case that it is singular in English, this is the translation that I would use.