Everton boss David Moyes has thanked Irvine for his service as assistant manager at Goodison Park, where he has spent the last five years.
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Moyes: "He did a very good job for us, and the opportunity to go to Preston was one that I wouldn't want to deny him.
"He knows I didn't want him to go, but we couldn't stand in his way.
"I would like to thank Alan for all the hard work he has done here and for all the help he has given me during the last five years."