Moyes has retained close links with the Deepdale outfit since he left for the Premiership five years ago.
Scot Irvine, 49, has waited 15 years for a management chance having also coached at Blackburn and Newcastle.
He said: "David is a very honest man and was terrific about it - I can't thank him enough.
"He spoke in glowing terms about Preston and didn't have a bad word to say about them.
"He felt it was a great opportunity for me."
After missing the play-offs by one place last season, Preston are now struggling fourth from bottom of the Championship.
Boss Paul Simpson was dismissed last week.