Entropy change for an irreversible isolated system is

From second law of thermodynamics TdS⩾δQ⇒TdS⩾dU+δW
For an irreversible process S−dU−δW>0. So, system undergoing irreversible change under constant internal energy and constant volume will have entropy change greater than zero dS>0.

And for an reversible process TdS−dU−δW=0.