McDowell and Casey flying high
McDowell made his tournament debut when Fota Island last hosted the event in 2002 and finished 27th, but that was just one of six halfway cuts the former US Open champion has made in 12 appearances.
The 34-year-old has never recorded a top-10 finish but carded an opening 68 on Thursday and moved into a share of the lead as he and Casey matched each other shot for shot in Cork.
Starting from the 10th, McDowell began with three pars before holing from 20 feet for birdie on the par-three 11th, only to promptly give the shot back on the next after failing to get up and down from a greenside bunker.
McDowell, who was joint second after the opening round of the US Open at Pinehurst last week before finishing 28th, bounced back straight away with a birdie on the 15th and safely negotiated the par-three 17th, a hole where he hit his tee shot so poorly on Thursday that it came up more than 50 yards short of the green.
A superb approach to the second gave McDowell his third birdie of the day and two more followed in quick succession, the Northern Irishman holing from 15 feet on the third and fourth to move to seven under par.
Casey had gone to the turn in 33 with birdies on the 11th and 16th, the former Ryder Cup player having also left birdie putts agonisingly on the edge of the hole on the 13th and 14th.
And the world number 83 kept pace with McDowell thanks to birdies on the first, second and fourth, making it a four-way tie at the top of the leaderboard with compatriot Robert Rock and overnight leader Mikko Ilonen, who was among the later starters.
Rock, who lost a play-off to Shane Lowry at County Louth in 2009 but claimed the first prize of 500,000 euros as Lowry was still an amateur, had gone to the turn in 34 and carded three birdies and one bogey on the back nine.
Given the standard of play, it came as a surprise when McDowell missed from six feet for another birdie on the par-five fifth, but he soon made amends from a similar distance on the next.
That was only good enough for a share of the lead however, Rock - who was joint runner-up behind Casey last year - having holed from five feet on the 16th to move to eight under.