Nathan Dyer forced his way through the visiting defence to tuck the ball home from a tight angle, with a Jonas Olsson slip in the penalty area proving costly.
Wayne Routledge doubled the advantage with a wonder-strike in the 24th minute, before Dyer rounded off the scoring 19 minutes from time.
Irvine lamented: "It couldn't have been a worse start, could it? You spend a week trying to get ready for a game, you develop a gameplan, and then somebody slips and that can't be legislated for.
"It makes what would have been a tough game an even tougher one and we certainly got rocked by that.
"We gave the ball away too cheaply, especially in the final third.
"We'd spent the time in the dressing room prior to the game talking about how important a good start was, and we got the worst possible start.
"As things were looking like they might just settle down for us, we made another mistake.
"Before you know it, you're 2-0 down and you're standing on the side desperate for half-time to come.
"Full marks to the players, they responded well to the half-time break and came out and made fewer unforced errors, kept the ball better and created some chances."
