Irvine told the club's official website: "It's absolutely terrific.
"I think everybody knows he has been in fantastic form, all the way through last season and certainly the start of this season.
"He's a very important player for us so I am absolutely delighted. He's probably one of the best goalkeepers in our division.
"He's a player who is continuing to get better, has a great future ahead of him and I'm delighted that it's with us."