Two people agree to work together despite their differences. They shake hands.
One of them says, "Allies." (As in, "let's be allies" or "we are allies now")

What would that person say if speaking Irish? Would it be plural? Vocative? Can it be just the single word like it is in English or do I need more?

Thanks!!

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I'm very much a beginner learner. Don't hesitate to correct me, as long as you explain why. This is how I learn best!

Gura comhghuaillithe dá chéile sinn. (May we be allies with another)

But that's perhaps a bit stilted.

or
Bímis inár gcomhghuaillithe. (Let us be allies)

Thanks, Labhrás. So "Comhghuaillithe" doesn't work by itself grammatically?

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I'm very much a beginner learner. Don't hesitate to correct me, as long as you explain why. This is how I learn best!

I see no reason it shouldn't. I'd say it works just the same as it works ( or doesn't, depending on the context) in English.
A slightly fuller version: Comhghuaillithe muid anois - "We are allies now."

I don’t see any reason it shouldn’t, either. But I don’t know.
In most languages such one-word utterances work, often with a special intonation (e.g. as if part of a longer sentence or question).

If it works grammatically, then I think I'm good. In context, this bit of dialogue is actually referring back to a previous longer thought.

Can anyone help with pronunciation? I can't find it on Forvo.

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I'm very much a beginner learner. Don't hesitate to correct me, as long as you explain why. This is how I learn best!

I think "cairde" (friends) would work too.

Comhghuaillithe - is now on Forvo

Abair and Teanglann are also good resources.

Oh wow! Thanks!!!

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I'm very much a beginner learner. Don't hesitate to correct me, as long as you explain why. This is how I learn best!

Would 'comrádaithe' be a good option here - 'comrades'?

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I would opt for "comhghuaillithe". One can be an ally without being a friend or comrade - as happened during WW2