The home favourite, 47, leads a posse of European stars who he is set to skipper at the 2020 Ryder Cup.

South Africa's Zander Lombard is one behind with England's Eddie Pepperell and Lee Slattery among those two back.

Those three off the pace include Northern Ireland's Cormac Sharvin, Ireland's Shane Lowry plus England's Ian Poulter and Lee Westwood.

Afternoon starter Harrington, now ranked 291st in the world having not won since the 2016 Portugal Masters, birdied both par threes on the outward half as he also picked up two further strokes at the sun-splashed county Clare venue

Following another gain at the 10th, a wayward tee shot at the short 11th saw him drop his only shot but an up-and-down from a greenside bunker saw him respond with an immediate birdie followed by another at the 13th.

The twice Open champion finished his day on a high as another superb escape from the sand saw him pick up a further birdie at the last as the 2007 winner completed his lowest-ever Irish Open round.

Harrington looked set to go into Friday's action with a two-shot lead but world number 316 Lombard finished with three successive birdies to cut the Irishman's lead to a stroke.

Harrington overhauls long time leaders

Earlier, Harrington's afternoon round saw him finally overhaul the group on 65, which included Australian Wade Ormsby, who was in the first group out at 06:45 BST.

A broken wrist at the end of last year kept Harrington out of action until March but after missing the cut in three of his last four events, Harrington regained his form on the links turf where he has excelled in the past.

"Links golf suits me because you can really work your way around," said the Dubliner, whose third major win came at the 2008 US PGA Championship.

"I had neutral expectations. I just wanted to take stock of my game and did not expect a 63 but it was a bit of a stroll in the park."

Ormsby, 39, whose only previous European Tour win was the Hong Kong Open in 2017, was joined on 65 by English pair Pepperell and Slattery plus Danish Ryder Cup player Thorbjorn Olesen, France's Mike Lorenzo-Vera and South Korea's Hyo-won Park.

Lorenzo-Vera and Olesen have already secured Open Championship places but Slattery, Ormsby and Park are among those battling for one of the three Royal Portrush spots available for top-10 finishers this week.

Lowry, who won the tournament as an amateur in 2009, recovered from an early bogey to card five birdies as his 66 left him in a nine-strong group that includes Poulter, Westwood and another Englishman Oliver Wilson plus Scotland's Robert MacIntyre.

Two-time major winner Martin Kaymer boosted his hopes of clinching one of the three Open Championship spots as he also shot a 66 while England's Tommy Fleetwood, Scotland's defending champion Russell Knox and 2017 winner Jon Rahm are all four behind Harrington.

Lowry has stroke of luck at 14th

Lowry had a stroke of good fortune on the 14th as his approach shot from the rough looked set to career over the green only to hit the pin and leave him with a tap-in birdie.

Despite an impressive season to date which included his Abu Dhabi Championship triumph in January, Lowry said the weight of home expectation at Lahinch left him feeling "uneasy and anxious" prior to Thursday's round.

"Over the years, I've tried to dampen it and take expectation off myself but it's just so hard," the world number 35 told Sky Sports.

"I was out on the putting green this morning and I just couldn't wait to get going."

Tommy Fleetwood's three-under-par 67 included a double bogey at the 15th

World number 20 Fleetwood had a blemish free opening 13 holes as he moved to four under after 13 but double bogeyed the 15th after three putting from 15 feet before notching an eight footer for birdie at the last.

The 2011 Open champion Darren Clarke, now playing most of his golf on the Champions Tour in the US, carded two birdies in his final four holes to sign for a one-over 71 which was one better than compatriot Graeme McDowell, who cut a frustrated figure during his round.

The 2010 US Open champion, using the Lahinch event to get in some links practice before The Open Championship in his native Portrush, dropped shots at the second and the third with his sole birdie coming at the eighth.