Can this be used in a sarcastic way or is there a different phrase used?
I'm not sure, but I'm guessing that it could be either depending on how you say it just like English.
I used it sarcastically with a native Gaelic speaker and they didn't correct me (though they may just have been going easy on me).
Good question, one that I had too, which is why I came to this discussion. I wish one of the moderators would enlighten us with an answer.
why do we use spòrsail for fun, but èibhinn for funny?
Why wouldn't we? What makes you think the words would be related?
Because the words are related in English. Doesn't mean that they have to be so in every other language, but they might be.
"spòrsail" (FUN) : "SPASS" ... "èibhinn" (FUNNY) : " MERKWÜRDIG / LUSTIG / SELTSAM" (... in German, methinks !)