Liis is correct, but I'd shorten it to Lost mey-rot, "Was (a) joke." Or, you could say Lost lo, meaning "Was (a) lie/deception," or Lost nok, meaning "Was (a) lie." The second one also means "was lying" as in "lying down," though, so I'd use Lost lo to avoid confusion.

Another phrase that could be useful is Orin brit ro, meaning "Fully beautiful balance," which is a general expression used to express irony. It could possibly be used when one is telling the truth and finds it to be humorous. (Odahviing says this phrase after being captured in Dragonsreach, during the mission "The Fallen.")