For an ideal gas, internal energy is a function of temperature only. Since the process is isothermal, the change in temperature is zero ($\Delta T = 0$), which implies the change in internal energy is also zero ($\Delta U = 0$) . For any irreversible process, the total entropy change of the universe (system + surroundings) must be strictly positive ($\Delta S_{total} > 0$), meaning $\Delta S_{total} \neq 0$ .