McIlroy and McDowell, inevitably attracting the biggest crowds, both birdied the fifth and seventh, but at five under and three under they were still in the pack.
Their fellow northern Irishman Michael Hoey, however, birdied the 384-yard second and on seven under was up to joint fifth.
Rory McIlroy made a nightmare start to his third round at the Irish Open in Killarney today.
Still with hopes of challenging for the title on four under par at halfway, six behind German Marcel Siem, the US Open champion double-bogeyed the opening hole in the windier conditions and then dropped another shot on the short third.
The 377-yard first hole runs alongside Lough Leane, and after his tee shot finished on the beach in amongst stones, McIlroy advanced his ball only about 20 yards.
His third only just made it onto the front of the green and from there he three-putted for a six.
Come the 200-yard third another poor shot found the greenside bunker, and by failing to get up and down he fell nine adrift of Siem.
Graeme McDowell, on his 32nd birthday, started with a birdie putt of nearly 20 feet to reach five under, but he was in the same trap as McIlroy two holes later and after a poor recovery three-putted for a double-bogey five.
Siem led by one overnight from Indian Jeev Milkha Singh and Dane Soren Hansen.