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
is takes a slightly different syntax than all the other verbs. Normally, Irish is Verb-Subject-Object. With
is, however, it's Verb-Complement-Subject.
So literally word-for-word we have "Are animals they horses". Since it's a copular construction, we take the noun phrase at the end and bring it to the front for English: "Horses, they are animals" or just "Horses are animals".