England’s Mark Foster charged into an early lead as Royal Portrush staged the first Irish Open north of the border since 1953.
With Graeme McDowell and Padraig Harrington, Europe's top two performers at the recent US Open, making good starts in heavy rain, Foster completed a brilliant front nine 31.
McDowell, runner-up to American Webb Simpson in San Francisco, birdied the long tenth and 210 yard 14th - "Calamity Corner" as it is known - to share the early lead.
Harrington, who came fourth at The Olympic Club, matched the Ulsterman's opening four and followed with four pars in a row.
Alongside McDowell on two under were Ryder Cup Captain José María Olazábal, Belgian Nicolas Colsaerts, England's Mark Foster, Welshman Bradley Dredge, Thai Thongchai Jaidee and South African Jbe Kruger.
World Number Two Rory McIlroy, who has missed four of his last five cuts, and Open Champion Darren Clarke, who has not made a cut all year and missed the US Open to rest a groin strain, were among the later starters.
At least there was no strong wind yet and rain was never going to deter the local fans from flocking in. A 27,000 all-ticket crowd - the first sell-out ever for a regular European Tour event - was expected.
As the weather improved, McDowell and Olazábal remained two under after eight and seven holes respectively, while Harrington was still one under, but Foster became outright leader when he picked up further strokes at the sixth and seventh.
On four under he was one ahead of Northern Irishman Michael Hoey and Australian Matthew Zions.
For the third time in under two months Olazábal was paired with Scot Paul Lawrie, close to securing a place in his team for September's match. The former Open Champion parred his first six holes.
Foster registered a fifth birdie of the day on the long ninth, but McDowell ran up a double bogey seven at the 581 yard 17th.
As he finished the inward half he was being watched by Northern Ireland First Minister Peter Robinson and his Deputy Martin McGuinness.
Harrington had birdied the 17th to be two under, but Foster led by two from Hoey, South African Keith Horne and Irish amateur Dermot McElroy.