"Tá" is always used for adjectives, while "is" is only used to identify. You can say "Is buachaill deas é", because you're identifying the boy as a "nice boy" rather than just describing him as nice. You could say "Tá an buachaill go deas" (not sure about the usage of "go" quite yet), meaning "The boy is nice" because the boy is being described as nice. There's a fine line between the two, but I hope this helped a little. I probably could've described it better... :/
To answer your original question, the NEID offers two suggestions for "sarcasm":
tarcaisne which the FGB translates as "Contempt, scorn; affront, insult" or
searbhas, which the FGB gives as "Bitterness, sourness, acidity".
As you can see, neither of them are entirely appropriate for describing my tongue-in-cheek response, which might be better described as magúil - " Mocking, jeering, jesting" :-)