Sheffield Wednesday manager refused to write off a 5-1 defeat at Exeter City as just a bad day at the office.
Four second half goals put paid to the Owls' chances of an away win in Devon despite a sparky performance from Jermaine Johnson, although the club stay second in the league.
Irvine said: "It was a very, very poor defensive display in the second half. We caved in after the second goal and you can't do that. The second half performance was totally unacceptable."
He added: "Prior to this, we had the second best defensive record in the league, we had only conceded 16 goals prior to today and we have conceded five in one game. It is just not acceptable to cave in, ion the way we did.
"We became very open as the game went on because we were trying to get back into the game and that doesn't help. But we still have to defend better. Exeter's second goal was a terrific strike, there is no question about it and these things happen. But it is important then to bounce back from that and we didn't do that.
"We were fully aware that we could have gone top. We have been in decent form from November, I hope it's a freak result. I don't work in a way where I will just write it off and say: "Never mind, we'll move on" because I do mind.
"I will look at exactly what went wrong and will try and do something about it. I don't think it is the right thing to do to just dismiss it as a bad day at the office."
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