Irish Open host Rory McIlroy has just started his opening round at Ballyliffin.
2007 winner Padraig Harrington is best of the Irish at the moment, he is four-under and one shot off the lead on the 18th.
Former US Open champion Graeme McDowell is next best after he carded a one-under par round of 71. Ryan Fox is the clubhouse leader on five-under.
Spain's Jon Rahm, who stormed to a six-shot win at Portstewart last year, made a solid start to the defence of his title at Ballyliffin from the 10th hole as he followed three straight pars with a birdie on the par-five 13th.
At one-under par the world number five was two shots off the early pace being set by England's Robert Rock and compatriot Adrian Otaegui, with another Spaniard, Rafa Cabrera Bello, part of a four-strong group on two under.
Playing alongside Rahm, McDowell was also one under par thanks to two birdies and a bogey in the first four holes.
McDowell had predicted the winning score could be 25-under par if the weather conditions remained ideal, but wayward shots were still being punished and three bogeys in the space of four holes saw the Northern Irishman reach the turn in one over.
Rahm was also on the same score after following a birdie on the 17th with a double bogey on the next, the 23-year-old duffing his third shot into a greenside bunker and slamming his club into the turf in frustration.
At the top of the leaderboard, Rock had moved into the outright lead on four under par thanks to a birdie on the ninth.
Rahm's bid to emulate his idol Seve Ballesteros by winning back-to-back Irish Open titles suffered another blow when he also double-bogeyed the first after driving into a fairway bunker and then compounded the error by three-putting from long range.
And although he did birdie the par-five fourth to get back to two over, that was eight shots off the pace as Rock picked up further shots on the 11th and 13th.
McDowell, who admitted on Wednesday that he is suffering from "scar tissue" caused by his recent struggles, had birdied the fourth and sixth to get back to level par.