"Captain of the team."
Translation:Captaen na foirne.
Not really. The Irish for "team" is foireann, or an fhoireann with the definite article (lenited after an, because it is a feminine word).
But in grammar terms, when you use "of" you have to use the genitive form of the noun (foirne), instead of the nominative (foireann). So you would say captaen foirne for "a captain of a team" (or "a team captain") and "captaen na foirne for "the captain of the team" (or "the team captain" - because, as if the genitive wasn't complicated enough already, the genitive uses na* for the singular article for feminine nouns).
The bottom line is that the "of" comes from the genitive form of the noun (foirne) and na is just "the*.
Leaving little easter eggs like this for people to find before they learn them seems to be a fun little trick Duolingo is used to using.
"de" (not "do") can mean "of" is the sense of indicating origin, so you can say something like "Is é duine de na leaideanna é" for "he's one of the boys", or "déan ceann de na rudaí seo a leanas" for "do one of the following", or "is scríbheoir den chéad scoth í" - "she's a writer of the best quality" (or "she's a first-class writer", but I wanted to highlight the "of the" in the translation).
But "Captain of the team" doesn't simply indicate origin - it is synonymous with "the team's captain", and so the genitive structure "Captaen na foirne" is more appropriate. "Ball den foireann" meaning "a member of/from the team" might be OK, but there's only one captain, so "captean den foireann" would not usually be used.