The pair finished one stroke ahead of world number three Rory McIlroy who carded a two-under 70 at the K Club on the outskirts of Dublin.
Warren conjured up seven birdies, including three in his last four holes, while Englishman Willett, the world number nine, made five birdies and four bogeys in an inconsistent round after his sparkling 65 on Thursday.
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"A little bit scrappy but grinding out quite nicely," Willett told the European PGA Tour website. "Marc's score today is a fantastic round of golf.
"It was a little bit trickier but as Marc showed, if you hit some good golf shots, there's a score out there."
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