Hi, I found this proverb: Coimhéad fearg fhear na foighde. Which is said to translate into: Beware of the anger of a patient man.
Do you use this for both sexes? Or is there an equivalent for women?

I would use this for both sexes...

If you put it as: Coimhéad fearg dhuine na foighde... it doesn't have the same 'béim'... but you wouldn't be wrong and you'd be understood.

You could say, if you really wanted to specify a 'certain' woman in your supposed 'generalisation':
Coimhéad fearg bhean na foighde... !! ... which wouldn't be wrong either and which would be understood - perhaps too well!!

As I said earlier, I would just use it as a general phrase.

Please wait for discussion on this... things may vary with different speakers and with different dialects...

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

Shouldn't we be using the genitive case for "fhear na foighde" - Coimhéad fearg fhir na foighde. ?

No, because you'd have a double genitive ("foighde" is genitive here). In such a case, the first word ("fear" in this case) is simply lenited.

Redwolf