"We had a good crop this year."
Translation:Bhí barr maith againn i mbliana.
It is probably more appropriate to consider i mbliana as a "fixed phrase", an idiom that is used in the vernacular. If you consider bliana the dative of bliain, then you would want to use it in uair sa bhliain or ag fanacht le bliain or bliain ar bhliain.
Note also, that i mbliana doesn't mean "in a year" (i gceann bliana, where bliana is in the genitive) but "this year".