Can anyone help me translate the slang "Badass", as in "that was badass" or "she is such a badass"?

Well, it won't translate directly, that's for sure! You'll be looking at a sentence, and it will probably be different in the first instance than the second. English slang rarely translates directly into other languages.

I can see "badass" having a bunch of potential meanings in English (anything from "really cool" to "really brave"). What meaning would you like here?

Redwolf

I'm figuring it is somewhere between "cool and "brave". It's certainly a positive attribute to do with being "bold" in the adventure sense of the word....

For a bad-ass [i.e. tough] person, you could use doscúch:
Is bean doscúch í [enlarged to show the accents better]
She's a tough woman

If you want to say that someone's "cool", you could use togha instead:
Is bean togha í
She's a cool/first-class/excellent woman
or
'Sí togha ar fad
She's way cool.

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

Thank you, Caoimhín, for following up. I appreciate it.

BTW, I live in the bay area also, in Berkeley.

Small world. I'm in Santa Cruz.

Redwolf

Or maybe just:

Dána

Use for men and women.

http://www.teanglann.ie/ga/fgb/dána