dream on

[keannu]

Does "dream on" have ironical, sarcastic nuance? It means "go ahead with your dream", but it seems to mean the opposite.

lt25
W : No, actually I've got free movie tickets for two.
M : Oh, are you asking me out on a date? Gosh, why am I so lucky?
W : Hey, dream on. My boyfriend is busy preparing for his exam. Anyway, will you come?

[SoothingDave]

Yes, it is sarcastic. It means "keep on dreaming," like this girl would only date this boy in his wildest dreams.