France's Grégory Bourdy gave the chasing pack a boost when he failed to build on a brilliant start to his Irish Open second round at Royal Portrush.
Joint overnight lead with Indian Jeev Milkha Singh, Bourdy eagled the long second and after ten holes had reached 13 under par - three clear of the field.
But then came bogeys at the 11th and 14th, both par threes, and the 30 year old's advantage was back down to one.
Italian Lorenzo Gagli was in second place on ten under after a 66 and Welshman Jamie Donaldson just one further back following a 67 that contained five successive birdies.
Home favourites Rory McIlroy and Darren Clarke both shot 69 to advance to five under and four under respectively.
For the World Number Two that prevented a fifth missed cut in his last six starts and on a course where he shot a record 61 as a 16 year old he knew he was capable of climbing in contention over the weekend.
Just to make the cut was important for Clarke - he has not managed that all season - but after closing with a 20 foot birdie putt he insisted there was no sense of relief because his sights are set higher than that.
His Open Championship defence is less than three weeks away and after a month out resting a groin strain he said: "It's important to have more competitive rounds under my belt - I'm just not tournament sharp.
"I started nicely today, but a bogey took the wind out of my sails and I couldn't give myself many chances after that.
"Conditions are getting harder and that's a good thing. Give these guys soft conditions and not that strong a wind and they will score low.
"But I've been surprised just how low it's been so far."
McIlroy, with girlfriend Caroline Wozniacki in attendance at the event after her Wimbledon first round defeat, said: "The last two days have probably been the best ball-striking rounds I've had for a while.
"I didn't do much wrong and hopefully I can have a good weekend. I need something around 64 or 65 to get myself into contention, but I've shot low scores here before.
"I'm hitting a lot of greens and if I take a few more chances I can turn a 69 into 65 very quickly."
Bourdy birdied the 17th for a 67 and two shot clubhouse lead, and again credited a practice round with Clarke on Tuesday for his impressive display.
“I didn't play so well at the start of the season, but I made a few good rounds,” said Bourdy, who is yet to register a top ten finish in 2012.
“I would like to just enjoy this week because it's a wonderful week. This week started very well on Tuesday, because I played with Darren in my practise round on this course. Darren has been great with me, he gave me a lot of advice, and maybe that gave me some confidence for the week, I don't know, but that's helped me for sure.
“I think because he knows the course very well, so on every shot he gave me an advice. Not many players will do that, so I asked a few times how he played a shot.
“The crowd is great - it's like in The British Open. I love the atmosphere because the crowds are clapping for every player, and no matter if you are French, Spanish or Irish; it's the same for every player.
“They know golf and they respect the game, and when you play good shots, they are clapping I think almost everybody the same, so it's great to see it. It's different at every tournament and we appreciate it a lot.”
Bourdy remained two ahead of Gagli in mid-afternoon, but Harrington continued to make a move. Birdies at the eighth, tenth and 12th took him into joint third on nine under with Donaldson, Mark Foster and Andrew Marshall.