(CNN) -- Englishman David Howell shot a seven-under-par 64 to lead the Irish Open but fellow countryman Robert Rock was disqualified for a scorecard error.
Richard Green of Australia, and Ireland's Damien McGrane, are one shot back from Howell, while Ryder Cup vice-captain Darren Clarke is part of a ten-strong chasing pack on five-under.
Northern Ireland's Rory McIlroy is three off the lead on four-under while three-time major winner Padraig Harrington is four adrift.
Howell didn't drop a shot at Killarney in a scintillating round, making birdies on the fourth, fifth and seventh to card a front nine total of 32.
He added birdies on 13 and 15 before an eagle on the 16th sent him into the lead with his lowest round of the year.
After his 64 Howell told the European Tour's official website: "No dropped shots. That was nice. Haven't done that for a long time but I holed a few bombs today.
"I chipped in and I think I holed three long ones as well. As it happens, I had a few more chances that I missed but I certainly holed more than my fair share, which in fairness you normally do when you shoot 64."
Howell's joy was in stark contrast to Rock's disappointment as he was thrown out of the tournament for signing a wrong scorecard after an opening round of six-under.
Rock got his scores on the 14th and 15th the wrong way round and was subsequently disqualified. He said: "It's my fault. I checked it, but didn't see it and it's my job to do that.
"I don't think I've been disqualified for anything before. I'd have preferred it to be after an 80! It's disappointing, I must admit."
Clarke, from Ireland, kept alive his chances of qualifying automatically for the Ryder Cup, despite already being named as one of European captain Colin Montgomerie's three vice-captains.
He hit every green in regulation and dropped just one shot on his way to a round of 66, the day after receiving an invitation to play in the year's final major -- the US PGA Championship, to be held in Wisconsin from August 9.
Clarke told the European Tour website: "I enjoyed it. It was great playing with Rory. We had perfect weather and big crowds and it was brilliant.
"People were saying how easy the course was going to be but after playing it more and more with the new greens, they are so undulating and it is tough to hole putts on them, so I don't think Killarney is going to that pushover course people thought it was going to be."