"The shirt is fine."
Translation:Tá an léine go maith.
'go' on it's own doesn't have any real meaning that I can think of off the top of my head. Irish in general doesn't use direct translation as it's a very poetic language. Direct translations are almost always incorrect. Take "Dia Dhuit" for instance. Directly translated it means "God be with you" which in english vernacular is shortened to "Goodbye" but "Dia dhuit" means hello
I'm sure you can make some quite direct translations from Irish into, say, Welsh. It's not some mystical language-of-poets quality that makes it hard to directly translate Irish into English (otherwise Irish speakers could say the same about English!) -- it's the fact that they're simply very distantly related languages, with little structure in common.
"well done" as in a steak, or "well done" as in congratulations?
One way to say "I like my steak well-done" is is maith liom mo stéig a bheith cócaráilte go maith, but that is closest in meaning to the somewhat ungrammatical "I like my steak to be cooked good" - "well cooked" would be the usual construction in English.
Generally go maith just means "good":
Níl an tsláinte go maith aici - "her health isn't good"
Níl mé go maith ag an nGaeilge - "I'm not good at Irish"
Tá sé go maith dá chuid páistí - "He is good to his children"
Sometimes it's "well":
d'iompair sé é féin go maith - "he behaved himself well"