The Baggies regrouped and took the lead once more through Saido Berahino, whose goal was cancelled out by Daley Blind in the dying minutes, but it was United's first that left Irvine particularly bitter.
"I looked straight away at [referee] Mike Dean to see whether he was going to give a foul or not," he told reporters.
"There's no doubt Marouane Fellaini bumped Joleon Lescott, while the ball was in flight. The fact that I looked at him, indicates I thought it might have been a foul.
"He didn't give it and we just have to accept that didn't happen and it was a great finish by Marouane."
Earlier, the West Brom boss conceded that the result left him feeling "bitterly disappointed".