Abdul Fatawu hit the winner in extra-time as the visitors won the Fifth Round tie 1-0.
Iraola, whose side had 27 shots on goal, said: "It is disappointing and it was a bad night for us because at the end it was the worst thing that could happen.
Advertisement
Advertisement
"We had a lot of shots and chances to finish the play and in the end if you play 120 minutes and don't score you deserve to go out.
"I think it's an opportunity missed."