Fox then two-putted for birdie on the 17th hole as Knox made the first of his enormous putts on 18 to set the clubhouse target at 14-under, giving the New Zealander a chance to claim the title if he could make birdie on the last.
A solitary blemish came on the 11th hole, but Knox quickly bounced back with a birdie on the 12th, before adding a further gain at the 15th to move in to a tie for the lead with Van Rooyen, Jorge Campillo and Ryan Fox - who had just dropped a shot on the 14th hole.
They went back down the 18th hole for the playoff and Knox pitched from 131 yards (120 metres) to virtually the same spot on the green. I'm so happy. I said (on the play-off hole) we've already made this putt, it can't be too hard.
"It was a little straighter than the other one, but it was tracking on a nice line and for it to go in, I feel bad for Ryan, but I guess it was my time". Each time Knox responded, with the pair the last ones standing in a round where several players launched bids at the title.
McIlroy will spend several days in London at the start of this week before stepping up his preparation for Carnoustie, where he made his Open debut as an 18-year-old and won the silver medal for leading amateur.More news: Giuliani: Trump Asked Comey To Give Michael Flynn 'A Break'
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"But I was able to hit a good shot and get on the green, which I was thrilled with".
South Africa's Erik van Rooyen began the day with a four-shot lead but struggled to a 74 to finish alongside Rahm on 12 under.
The win is likely to lift Knox inside the world's top 50 as he prepares to return to home soil for next week's Aberdeen Standard Investments Scottish Open at Gullane.
All three hunted down Van Rooyen.
Russell Knox's hopes of making this year's European Ryder Cup have received a significant boost with victory in the Irish Open. That would certainly make up for his disappointment at missing the last Ryder Cup, when he came so close on his own merits, but then failed to get one of the captain's wild card picks. Instead, the long hitting Kiwi will leave Ballyliffin in Ireland with mixed feelings after losing a play-off to Scotland's Russell Knox. I mean, how do you say you tried too hard?