Hi, mates!

A little question - Is it correct translation of "Lucky 13"? -> Trí déag ádhúil

Thanks in advance.

What you have there is an attempt to do it in Irish, not Scottish Gaelic. In either language, in order to be clear that you are talking about the number itself, I think you'd need to add the word "number", so in Gaelic you'd have:

àireamh a trì deug fortanach
lucky number 13

I put it in large type so that you can see the accent marks, and note that in Gaelic they run in the opposite direction to the way they run in Irish (i.e. slanted down to the right, as an "accent grave", to use the French term). Also, in Gaelic many words are spelled a bit differently from their forms in Irish, as is the case here with "deug" (Irish "deag").

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