My grandmother use to say Harm None. can someone please give me a accurate Irish/Scottish Gaelic translation of Harm None

You can't really say it quite so briefly in Gaelic:

This works as a slogan or if you are speaking directly to a single person with whom you're on familiar terms (as with the French "tu" or German "Du"):

Na goirtich duine sam bith
Don't harm anyone

If you're saying it to a group of people or to someone whom you should address more formally (as with the French "vous" or German "Sie"), then it would be slightly different:

Na goirtichibh duine sam bith
Don't harm anyone

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