Sheffield’s Willett carded eight birdies and a solitary bogey at the K Club in a seven-under-par 65 to finish two shots ahead of McIlroy, who has missed the cut in this event for the last three years.
South Africa’s Jaco van Zyl and England’s Callum Shinkwin were a shot further back, with defending champion Soren Kjeldsen, two-time major winner Martin Kaymer, 2010 champion Ross Fisher and Brett Rumford on three under.
Fisher and Kaymer were the only players in the first half of the field to break 70 as heavy downpours lashed the 2006 Ryder Cup venue before giving way to bright and breezy afternoon conditions.
“It did not look pretty when I opened the curtains,” said Willett, who missed the cut by a shot in the Players Championship last week in his first event since Augusta. “Three under (yesterday morning) was a fantastic score and to be able to do four better (in the afternoon) I’m very happy.
“Last week didn’t quite go as we planned, but we were still a little bit tired, a little bit rusty with all the things we had to do. This week has been trying to get back to what I do and try to get grinding and it’s obviously paid off thus far.
“I got off to a pretty scrappy start, but got a bit hot with the putter on the back nine and holed a lot of footage. You can’t really go out there and expect to shoot any kind of number and 65 was far beyond my expectations (for yesterday).”
McIlroy has recorded just two top-10 finishes in nine appearances in the Irish Open and was frustrated at what he felt was “negative spin” being put on his failure to win so far in 2016.
However, the 27-year-old admitted the prospect of getting that win in front of his home crowd gives him “goosebumps”, adding: “To win here, no matter what the circumstances are, whether I’ve won the week before or whether I haven’t won in six months, would be something very special and I’ve got off to a great start towards doing that this week. Just have to keep playing the way I did (yesterday).”
Lindrick GC’s Luke Jackson is one shot off the lead after two rounds of the PDC Golf Championship on the HotelPlanner.com EuroPro Tour at Frilford Heath GC.
Jackson shot 70 68 to sit just off a three-way tie at the top held by Laurie Canter, Luke Cornford and Dave Coupland.