"A man wants food."
Translation:Teastaíonn bia ó fhear.
If the following noun is without an article, then these prepositions will lenite the noun: a, de, do, faoi, mar, ó, roimh, trí, and um. There are particular rules for the prepositions ar, gan, idir, and thar; sometimes they’ll lenite the following noun, and sometimes they won’t.
Teastaionn fear bia doesn't mean anything in Irish, but if it did, it would be closer to "food-man is wanted" than "a man wants food".
The subject of the English verb "want" is the thing/person that is doing the wanting ("a man"). The subject of the Irish verb teastaigh is the thing that is wanted ("food", in this case).
ó fhear generally means "from a man", but in this case it means "by a man". About the only time that ó would mean "of" is in a phrase like "out of" (ní raibh sé ó bhaol go fóill - "he wasn't out of danger yet"), though there are bound to be examples where "of" and "from" overlap enough in English, that ó could be interpreted as "of".