India's Jeev Milkha Singh and Gregory Bourdy of France led the way after the Irish Open first round while world number two Rory McIlroy was five shots off the pace after making two late errors on the greens.
U.S. PGA champion Keegan Bradley struggled to a 73 in his first competitive round in Europe while
The appearance of Northern Irish trio McIlroy, McDowell and Clarke and Irishman Harrington at the Royal Portrush links course in County Antrim has helped the European Tour to achieve a first by declaring a sellout for all four days.
Former U.S. Open winner Grame McDowell of Northern could only post a 71. McIlroy, watched by girlfriend Caroline Wozniacki following her first-round defeat at the Wimbledon tennis tournament, was four-under for his round until he three-putted the 16th and 18th to finish with a 70.
McIlroy has missed the cut in four of his last five outings.
Jose Maria Olazabal of Spain carded a 69 to stay four shots off the lead while Englishman Symon Dyson had a round of 67, three behinb the joint leaders.
Triple major champion Padraig Harrington was among a group of 10 players on 67, including McDowell.
Singh and Bourdy shrugged off the rainy, windy conditions to shoot matching seven-under 65s as the tournament returned to Northern Ireland for the first time since 1953.
Britons Mark Foster and Andrew Marshall were in joint third place on 66 alongside Frenchman Edouard Dubois, Finn Mikko Ilonen, Swede Oscar Floren and Australia's Matthew Zions.
Reuters