Hugo is correct – “theas” isn’t a noun, so you can’t say “sa theas” – it just doesn’t make any sense in Irish.
Apart from the grammar, from a phonological point of view it’s also uncomfortable to say, because words that start with “t” don’t take a séimhiú after “sa”, (you say “sa teach” or “sa tuaisceart”, for example), so “sa theas” just feels wrong, (even though the “h” in “theas” is just a normal part of the word, and “theas” isn’t the noun “teas” (heat/warmth) with a séimhiú).
You can say:
Tá sé ina chónaí theas i gCorcaigh.
Tá Corcaigh sa chuid theas den tír.
Tá Corcaigh taobh theas de Tiobráid Árann.