The DuoLingo translation uses the word "seriously" which is an adverb. The hover helps show the word as an adjective, "serious". Which is the correct way to say "seriously" in Portugues?
Seriamente. But this sentence really fits well with "sério".
Would "no, en serio" be a valid translation for "no, seriously"?
Yes, that's why I was wondering that.
In Portuguese that doesn't work......
"A sério" works though
When would this expression be used in Portuguese? I think the English version here would perhaps be used ironically or sarcastically more often than the "direct" version. Is that true in Portuguese as well?
It depends on the intonation. It can be either a "direct" way or an ironic one.