Here's a bit of solid sarcasm in the Bible, when Elijah is challenging the prophets of Baal to call down fire from their god:

1 Kings 18:27:

At noon Elijah began to taunt them. “Shout louder!” he said. “Surely he is a god! Perhaps he is deep in thought, or busy, or traveling. Maybe he is sleeping and must be awakened."

While the events may not be historical, of course, the text dates to the 7th or 8th century BCE.

Someone with a better grip on ancient writings can come in and give a better example I'm sure, but for my own area of medieval literature, I can give one small example.

Around the year 1000, Iceland's Alþingr (early parliament) was debating on whether or not to convert to Christianity. The Christne Saga, written around the 12th century, has this tidbit during those debates:

Then came a man running to the Alþingr and said that earth-fire was coming up in Olfuse, and had come to the house of Þóroz the goði. Then took the heathen men to saying 'This is not to be marveled at, as the gods are angry at such speeches.' Then said Snorre the goði: 'About what were the gods angry when where we stand now was burning lava?' After that went men from the Rock of Laws.

I don't know if this is a perfect example of sarcasm, since so much of sarcasm is in the voice inflection; this could have been simply a statement. I've always thought it was interesting though.