"Mice are animals."
Translation:Is ainmhithe iad lucha.
for what I've understood the pronoun is integral part of the copula: "Is <subject complement> mé/tù/sé/sì/muid/sibh/siad " is like a conjungation in Standard Irish: you can't replace the pronoun. So the logical subjec comes after the pronoun.
There's a complete explanation with several examples: https://www.duolingo.com/comment/4300985
For a comparison between English Copula and most of other copula structures: http://en.wikipedia.org/wiki/Copula_(linguistics)#Irish
There are two major divisions of Irish copular statements: those which identify, and those which classify. The two types are structured differently, depending upon the particular type of subject and the particular type of complement. The “complete explanation” page discusses only a classification statement with a definite subject (i.e. a “The X is a Y” statement); for a truly complete explanation of the several varieties, see here.
This sentence is a classificational statement with an indefinite subject (i.e. a “An X is a Y” statement), and thus does not require a pronomial sub-subject; the iad in the suggested translation is superfluous.
I wasn't sure whether "it's not redundant in Irish" was meant generally or specific to this exercise particularly since another seemingly very knowledgeable contributor is strongly arguing that it isn't required here and you didn't appear to have contradicted his assertion. Thanks for the quick reply.