Jon Walters saw his penalty saved by Andy Lonergan but converted the rebound.
Preston boss Irvine said: "I thought it was a poor decision (the sending-off).
"We're told you have to make your body bigger - that's the criteria for it to be a red card.
"But Callum had his arms in front of his body. It was a natural reaction when someone smashes the ball at you from four yards."
Irvine did, however, heap praise on his side for the way they hung on with 10 men after the break, pocketing a hard-earned point.
"They're a terrific group of players," he said. "They know their limitations but they also know they'll get rewarded for working hard.
"They're prepared to do the horrible jobs to make sure we stay in games.
"We spoke about it at half-time and said things could go one of two ways. We could either feel sorry for ourselves and be a bit angry about what had happened or go out and get a good result with one man short.
"That was what we did and if we'd chosen our passes better on the counter-attack it might have been a different result."
Preston started brightly and should have been in front within three minutes, when Jon Parkin raced behind the home defence but saw his low effort brilliantly saved by Richard Wright.
However, the Lilywhites were soon ahead when Wallace received the ball wide on the left and twisted inside Alex Bruce.
He drilled a low shot which flashed across goal and in off the far post.
Ipswich fought back and finished the first half strongly. They were level just before the break when Davidson handled a shot from Liam Trotter, after goalkeeper Lonergan flapped at a harmless-looking cross from Jack Colback.
The left-back was sent off, and although Walters' penalty was saved, the midfielder was able to convert the rebound to make it 1-1.
Town boss Roy Keane said: "We huffed and puffed and brought all our three substitutes on to try and win the game.
"But I can't have any complaints about the result. If anything, Preston looked more dangerous on the break.
"I can't fault my players but I suppose this game shows why we're desperately trying to add to our squad.
"At the moment we're nowhere near but I'm expecting a busy couple of days.
"I certainly won't be putting my feet up and hopefully I can bring three, maybe four, players in before Tuesday.
"I have a very good idea of the areas I want to strengthen in and there's a lot of hard work ahead."