Struggling West Brom lost their fourth Premier League match in a row despite Craig Dawson giving them a 10th-minute lead.
Headers from Kevin Nolan and James Tomkins put the visitors in front before the break and they held on despite a second-half West Brom barrage.
West Brom's Victor Anichebe had a hatful of late chances but could not find a way past goalkeeper Adrian as the hosts' woes continued.
"It's bitterly disappointing as we started the game pretty well and got ahead," Irvine told BBC Five Live.
"We made a couple of mistakes and people started getting nervous. When the equaliser came, I felt we could deal with that at half-time.
"The second goal made our job harder.
"I thought [in] the second half, the performance was very, very good and we deserved to get something out of it tonight."
West Brom sit 16th in the table but their next three matches - against Hull City, Aston Villa and Queens Park Rangers - come against sides also in the bottom half.
And Irvine said he is aware it is a crucial period - both for his side and his future.
"We have Hull, Villa and QPR coming up," he added.
"They'll be difficult games but they are games... we need to go and be like where we were at the start of the game tonight, like [we] were in the second half."
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