^ I agree with that (though the prepositional version doesn’t sound unnatural to me, just not quite as nice as the genitival version). I also think suaimhneas síoraí dá anam is idiomatic and set-phraseistic (new word!) enough that we shouldn’t break it up.

Since this appears to be for a tombstone of some kind, I’d suggest cutting out the verb altogether and simply using two lines:

Fear an Chlocháin Bhig
Suaimhneas síoraí dá anam

‘The man from Cloha(u)nbeg
May he rest in peace’ [lit. ‘eternal peace to his soul’]

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.