Harrington, who is currently ranked 268th in the world and without a victory since 2016, carded an eagle, three birdies and a solitary bogey at Ballyliffin to finish four under par, a shot behind New Zealand’s Ryan Fox.
The three-time major winner’s score was matched by six players, including fellow Ryder Cup vice-captain Lee Westwood and former Masters champion, Danny Willett, while tournament host Rory McIlroy carded a 70 and defending champion Jon Rahm struggled to a 74.
Harrington has missed the cut in seven of his 12 events in 2018 but was surprised at the start of the week to find himself rated a 66-1 chance – or higher – to demonstrate his prowess on links courses and lift the trophy in Donegal.
“There’s not often there’s value in my game,” said the 46-year-old, who won the Irish Open in 2007, two months before claiming his first major title in the Open Championship at this year’s venue of Carnoustie.
“The bookies in Ireland would make sure they cut you to a low price knowing that sentimentally people would back you. The bookies don’t normally get it wrong, but to put me out there at 66-1 on a links golf course in my home country was strange on their part.
“My performances always go up a level or two when it comes to a links course. Maybe they factored that in and that says I’m even worse than I thought!”
Willett reached a career-high of ninth in the world after his victory at Augusta National in 2016, but has slumped to 442nd in the rankings after suffering numerous injuries and a loss of form.
“The game wasn’t far off last week,” said Willett, whose missed cut in the French Open was his ninth in 12 events this season. “I went home early and worked hard over the weekend and came out pretty fresh.
“We know the work is in there and we’re doing the right work but it’s always nice to get these scores.”
McIlroy was frustrated not to capitalise on some superb play from tee to green after reaping the rewards of some hard work on his swing with his coach Michael Bannon after Wednesday’s pro-am.
“It could have been a lot better,” McIlroy said. “It’s the best I’ve hit the ball in a long time. I just struggled on the greens. It was either I hit good putts and just didn’t read them right, or sometimes it was maybe a reaction of putts I had hit previously on a couple holes earlier.
“I would love to be able to say I’m saving them for a couple of weeks’ time [in the Open] but I’ve got to take the positives. I’m not walking away satisfied but I’m very happy with one part of my game and the positives outweigh the negatives at this point.”
Rahm’s 74 included back-to-back double bogeys around the turn, while playing partner Graeme McDowell birdied three of his last six holes to finish one under.
“It was a grinding performance but nice to turn a 75 into a 71,” McDowell said. “It was one of those days that could have got away from me so I was happy to keep it under control and post something half decent.”