Translation:She has green eyes and she has blond, curly hair.
If I have "blond curly hair on me", it's clearly someone else's blond curly hair, because my hair is neither blond nor curly.
tá gruaig fhionn, chatach uirthi is only talking about her hair - the hair that she "has" in English, not someone else's hair that might might be "on" her.
Colours occur on a spectrum, and there aren't sharply defined boundaries between colours on the spectrum, so sometimes the names that are used for particular colours in different languages overlap, and don't line up exactly. The colour glas can also include blue-grey hues that wouldn't be described as "green" in English.
This is the entry for Glas in Dineen's 1904 dictionary:
glas, glaise, a., green, verdant (as grass); grey (as a horse or cloth); grey, bright, lustrous (of the eye); cold, inclement: lá glas, a rough, cold day; glas-aimsear, rough weather; bluish grey, as the sky; glas caorach, of natural colour, undyed (as wool).
Which earlier translation are you referring to?
In this exercise, there are two different clauses because two different prepositions are required - you use tá ... ar to say that someone "has hair" - tá gruaig fhionn uirthi, not tá gruaig fhionn aici, so the two clauses can't be combined into one in Irish. You could combine them in English, but Duolingo generally prefers that you translate the sentence as written, unless there's a compelling reason not to, and as there isn't anything particularly strange or unusual about "She has green eyes and she has blond, curly hair", that's the expected translation.