Dia dhaoibh!

So I've been trying my hand at translating some phrases for myself but just don't have the confidence in my Irish to be sure. I've only been learning for around 3 years and would like a more experienced opinion of a phrase.

"The hardest battles are fought in the mind."

My own take was "Throid na cathanna is crua san intinn."

I originally wanted to translate it into Old Irish but not sure if that is possible or within my means!

Your sentence translates as "The hardest battles fought in the mind"
Your phrase is active voice (battles fought) but what you want is passive voice (battles are fought)
And your phrase is past tense (they fought) but what you want is present tense (are).
In English the form "fought" is used in active and passive, active past and passive present, that's the reason for the confusion, I guess.

In Irish, passive voice isn't so prominent as it is in English.
It is possible only in three very special shades of meaning:
progressive:
Tá na cathanna is crua á dtroid san intinn. = The hardest battles are being fought in the mind
perfect:
Tá na cathanna is crua troidte san inntin. = The hardest battles have been fought in the mind
prospective:
Tá na cathanna is crua le troid san intinn. = The hardest battles are to be fought in the mind

All of them are somewhat clumsy and don't convey the more generalized meaning of "are fought"

Very often autonomous forms are used instead of passive voice in Irish.
And they are often the best way to translate English passive.
Troidtear é = one fights it -> It is fought
Troidtear na cathanna is crua san intinn. = One fights the hardest battles in the mind = The hardest battles are fought in the mind.

So, I'd recommend the last.

Yes, I second that

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

Ahh yes, that does make a lot more sense. I knew mine sounded weird when I said it out loud but I just couldn't wrap my head around how to word it otherwise. It figures the verb tense that was correct is the one I'm really the least familiar with using!

Many thanks!

On a related note, is "cuimhne" more in relation to one's memory in typical usage in Irish or is it also synonymous with mind as well?