Irwin is rated United's greatest modern day left-back.
"He is going to be a fantastic signing for years to come," said Irwin, who played 529 times for United, to Sky Sports.
"He showed he has great pace for England at the World Cup and in the warm-up games.
"He has always been great going forward, we have seen that in the Premier League.
"There was talk of Chelsea being in for him so I am delighted that he signed for United.
"He is a fantastic player, who will improve and get stronger."
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