"I am bad."
Translation:Táim go dona.
I should have pointed to a different section of Gramadach na Gaeilge - "Usage of adjectives"
Some subjective adjectives of assessment take the adverbial partigle go when used with the verb bí (they're kind of seen as "adverbs of the verb bí")
Tá an fear go maith - The man is good.
Tá an cailín go hálainn - The girl is beautiful.
Tá an lá go deas - The day is nice.
Tá an aimsir go dona - The weather is bad.
Tá an samhradh go haoibhinn - The summer is splendid.
Tá an rós go breá - The rose is pretty.
Tá sé go holc - It is evil.
Tá sí go hiontach - She is wonderful.
That can be read as a "Because that's just the way it is" answer. I don't know of any other explanation.