If Irvine is right, his team will need a further 11 points from five games after damaging Bristol City's hopes.
Nicky Maynard's header put City in front after 59 minutes but Preston responded and centre-back Youl Mawene grabbed an equaliser 10 minutes later.
Irvine admitted that neither he or City boss Gary Johnson had wanted a draw, which left Preston seventh and their opponents a place further back.
But he added: "Gary Johnson will probably feel it is a worse result for his side because we have maintained our three-point lead over them and one more game has gone by.
"Two months ago I thought it might take 75 points to get into the play-offs, but some of the leading sides have slipped up a few times since then and now I think 73 would give us a good chance.
"I look at it that if we can win all five of our remaining games, three of which are at home, we will have a great chance.
"I set the team up today to go for a win, but I think a draw was a fair result. All our remaining fixtures are tough and while I think we are capable of winning any match of this level, we are equally capable of losing them.
"The next one is the small matter of a local derby against Blackpool and we will be focusing all our attention on that."
A corner conceded under no pressure by City full-back Bradley Orr led to Mawene's volleyed equaliser and left City manager Johnson hugely frustrated.
"We were a goal away from a good performance," he said.
"But a couple of mistakes cost us dear and now our play-off chances look slim.
"While it's still mathematically possible for us to finish in the top six we will be giving it all we've got. But we can't afford any more basic errors.
"You'll have to ask Bradley what was going on when he conceded a corner.
Then Dele Adebola failed to stick to his man and allowed him to score.
"We played the sort of football I want and I felt we should have been ahead at half-time. There were chances in the second period, but the ball didn't quite break for us where it matters most."
City went in front through good work by Dele Adebola, who beat a defender on the left before providing a perfect cross for Maynard to head home.
But Preston were well worth their point and hit the outside of a post three minutes later through Stephen Elliott's low drive.
Mawene then stretched to volley Ross Wallace's left-wing corner past Adriano Basso for the equaliser, leaving his manager delighted, but surprised.
Irvine said: "Youl gets a few goals, but not all of them with that quality. It was a great finish."