The Englishman became the second player at this year's tournament to set a new low round at the Fota Island Resort after leader Mikko Ilonen carded 64 on Thursday.
Willett was seven shots adrift of Ilonen after two rounds in Cork, but his ace on Saturday accompanied six birdies to leave him leave him 11 under.
The 26-year-old, who also sunk a tee shot at the China Open in April, achieved the feat at the par-three seventh, having already gained a shot at the fourth.
He picked up another at the eighth, but it was a run of four birdies in the last five holes that really put him in contention.
"We had a perfect number, 168 yards with an eight iron but you never expect it to slam dunk," Willett told the European Tour's official website. "That was a bit of a bonus."
Leader Ilonen had a somewhat mixed day as four birdies were partially negated by two bogies.
Having dropped a shot at the 17th it looked as though he would share the lead with Willett heading into the final day, but the Finn restored his outright lead with a birdie at the par-five 18th.
Graeme McDowell finished strongly by gaining shots at the 16th and 17th, and sits alone in third on 10 under, while Frenchman Romain Wattel is a further stroke back and remains in contention.
Wattel is in a four-way tie for fourth, as Kristoffer Broberg and Matthew Baldwin both shot 66 to lift themselves up the leaderboard, while Fabrizio Zanotti went one better with a six-under-par 65.
England's Robert Rock started the day just two shots behind Ilonen, but he slipped to a tie for 21st after enduring a miserable day, including a double-bogey seven at the 10th, and three further dropped shots in the final four holes.