I am writing a mystery story and I really want to add a few phrases in Irish Gaelic since one of my main characters is a first-generation Irish-American. What I want to say is "He is not human. He does not feel." And I am using the "not human" in the sense of he is emotionless.
I have had Google Translate the sentences and got "Níl sé daonna. Ní bhraitheann sé." But I really want to double check the grammar and MOST IMPORTANT to me, that it is how a native speaker would say it. I know there are multiple ways of phrasing things based on the area of Ireland, and I really don't have a preference except maybe Standard Irish. If you need any clarification ask.
Please and thank you!

The word daonna means human, literally, so that's probably not a good choice for what you want, and the first clause is ungrammatical anyway. The second clause does literally mean "He does not feel", but it seems a bit stilted as a stand-alone expression, as though it lacks an object (and the verb mothaigh seems like a better choice for "feel" in this context), but here's a suggestion as to a brief way to say it:

Is anduine é.
He is inhuman.

Tá sé gan chroí.
He is without a heart. [This is a traditional way to say that someone is unfeeling]

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Perhaps:
Ní neach daonna atá ann (It is not a human being that is in him, aka, that he is)

I like it, at least

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ЯГОН ТОҶИК НЕСТ ИНҶО???

I like it too.

Or even just: Níl sé daonna.

Thank you everyone for your advice. I can see that getting the translation just right might be a bit more difficult than I first thought. But I really want to include it, so I'm going to keep trying. Once it seems like any kind of consensus is reached on the phrasing, then that's what I'll use.
Thank you again. And if anyone has any questions or needs clarification please ask. I'll try my best to answer any questions to help get the best translation possible.

There is nothing wrong with any of these translations. It is just whatever you feel like.
I wouldn't say there is any need to wait for consensus, since there is no disagreement about any of the forms being correct or suitable
Really just pick whatever you fancy, maybe whatever looks nicest, because all suggested options are satisfactory

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ЯГОН ТОҶИК НЕСТ ИНҶО???

Agreed, although (as the first one who responded), I'd go with what Cumhaí suggested and Bríd seconded for the first part. Bríd's a native speaker, and even to my non-native ears it does sound very Irish in structure.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.