The Dubliner, 37, bogeyed two of his last three holes to open with a disappointing, two-over par 74.
England's Justin Rose and South Africa's Trevor Immelman carded four-under par 68s to set the clubhouse target in perfect conditions
But Harrington was disappointed with his driving and his putting as he pencilled in two birdies against two bogeys and a double bogey six.
He said: "Bogeying the last there, missing a short putt, meant it was a disappointing day. I missed a lot of chances and I struggled off the tee, which are probably two of the more important things to do on this golf course.
"In the middle of the round I missed a lot of birdie putts and I think I got frustrated at the end. I could have been a couple under par coming to the last couple of holes.
"Instead, when I three-putted the 16th, I went over par and didn't feel like I should be and it probably got to me a little."
Harrington played the first conservatively, taking a three-wood off the tee and two-putting safely for par before picking up a birdie at the second.
Two three-woods left him just 34 yards short of the pin and he pitched to no more than a foot to get into red figures.
He saved par with a six-footer on the next after his pitch failed to climb the slope fronting the green, but then double-bogeyed the 455-yard fifth after driving into a bush and being forced to take a penalty drop.
In the end he did well to limit the damage to a double bogey six with a 10-foot putt but his momentum was gone.
Seven pars followed as he mixed poor driving with good scrambling and some nervous putting before he pitched and putted for birdie at the par-five 13th to get back to level par.
But he suffered a disappointing finish with three-putts from the front of the 16th sending him back to one-over.
A poor approach to the 17th, which finished 40 yards left of the green, left him facing a tough pitch.
But he fired it to just four feet and holed the putt to remain close to the lead before another pulled approach cost him at the last.
Pitching from the swale left of the green, Harrington's ball shaved the hole but finished four feet past.
But he pushed his par-saving effort to fall six shots off the early lead after a round that featured 31 putts and just six of 14 fairways in regulation.
Harrington added: "I probably got a bit frustrated with myself at the end but there is a still a long way to go - 54 holes. I'd rather hole the putts on Sunday than on Thursday."