The couple, there to watch the new production Peter Pan in Scarlet, also tried out new seating in the theatre's revamped auditorium.
The play, based on the novel by Geraldine McCaughrean, is the first production to be performed there since the work.
Mr Irons, who married Ms Cusack in 1978, said: "Theatres are not about seats, they're not about lights, they are about what happens in the auditorium, and to make something really happen in an auditorium you need to have productions being made in that theatre.
"What is important is that this theatre is a producing theatre again.
"If it does become a production house as well as a receiving house, it will I think surprise you with the life it gives, not only to itself but to the city.
"A city like Oxford needs a good theatre producing its own work."