The Portugal international has been linked with a move back to Old Trafford this summer, with his future seemingly unresolved at the Santiago Bernabeu.
Irwin believes that a deal could happen and will be affected by the outcome of Gareth Bale's potential move from Tottenham Hotspur to Madrid.
"So Manchester United have been linked for a second time this summer with a return to Old Trafford for Cristiano Ronaldo," Irwin told Ireland's Sunday World newspaper.
"Of course it could happen, but I believe the speculation may be sparked by the ever-clearer indications that Gareth Bale is on the way from Spurs to Real Madrid. Can Real Madrid, rich and all as they are, afford to buy and then pay that brilliant pair of players?
"If the answer is yes, then Madrid see them as their answer to the [Lionel] Messi and Neymar duo at Barcelona. If the answer is no, then Ronaldo will be allowed leave, and Manchester United is an obvious destination."
Ronaldo has two years remaining on his current contract with Madrid.