I know it's an idiom and idioms don't necessarily translate but i was wondering if "Toil iarnaí" might be a suitable translation for the phrase "Iron will"?
Go raibh maith agaibh, a chairde!
Seán Beag

As you have stated, it's an idiom. I wouldn't choose to use it. Why 'iron' and not 'steel' in Irish, or something else along the lines of 'unmoveable/indefatigable'?

I'd say something like rúndaingne (strong-mindedness).

De Bhaldraithe uses a hill, offering this for "He has an iron will": Tá sé chomh daingean (san intinn) le cnoc.
He also offers for "man of iron" fear gan trócaire and croí cloiche, but those both look to me to be closer in meaning to "merciless" or "hard-hearted".

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

"He's a steadfast (in mind/intent) as a hill"

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Yes, of course, I was reading cnoc but seeing cloch.

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

Yup, done that myself on more than one occasion.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Haha I'd read it as cloch in the original post as well, I guess the mind sees what it wants to see!
Thanks for the help, guys.