Translation:Tá áthas orm.
I think that might be a translation of the English phrase put with Irish words. In Irish you say that an emotion is "on you," rather than you "are" any certain emotion.
This also follows the VSO (Verb Subject Object) word order found in Irish. So, literally, I am happy would be said "happy is on me," which is why we get:
Tá athas orm, which follows tå (is) athas (happy) orm (on me)
I hope that helped!
Tá mé sásta is closer to "I am satisfied", which can be the equivalent of "I am happy" in some cases (Ta mé sásta le sin - "I'm happy with that"), but it's not as positive/emphatic as tá áthas orm.