Home favourite Harrington, 47, had eight birdies and a lone bogey to sit a shot ahead of South Africa’s Zander Lombard, with a group of seven players tied for third at Lahinch Golf Club.

“I like playing links golf. I knew coming into this run that if I get my head in the game I have an advantage,” said Harrington, participating in only his second European Tour event of the season following a broken wrist.

“Today, I did a lot of good things, holed a few putts here and there and got a few breaks. I didn’t expect 63 out of it. It would definitely be up there with one of the simplest, easiest, most pleasant days ever on the links.

“I know I don’t have to play as well as that for the next three days. It would be nice if I did but we’ll go out there and try and do more of the same. If nothing good happens for the next three days, so be it. I know I’m on the right track.”

Harrington, the 2007 champion, fired three straight birdies starting from the eighth hole but had a setback at the 11th where he missed the green for his only bogey of the day.

However, the three-time Major winner recovered to add further gains on the next two holes and finished with another birdie on the final one, much to the delight of the crowd.