The story was unrealistic and I couldn't believe it.
He seemed to be immersed in a world of unreality.
非現実的 is the adjectival form meaning 'unrealistic'. 非現実 is the noun form referring to the concept of unreality itself.
Compound of 非 (hi, 'non-') and 現実 (genjitsu, 'reality'). The prefix 非- is a Sino-Japanese morpheme meaning 'not' or 'un-', attached to nouns to form antonyms.