Van Rooyen began the third day in a three-way tie for the lead, but stormed to a superb 66 to finish 14 under par, four shots ahead of New Zealand's Ryan Fox and Sweden's Joakim Lagergren.
Among the early starters at Ballyliffin on Sunday were Ulstermen Graeme McDowell, Simon Thornton and Rory McIlroy - all too far behind to make a threat on the leaders.
McDowell, playing with Paul Dunne, found a birdie on the par 5 fourth - his third at the hole this week to put him one under for his round and level overall.
Dunne had back to back birdies on three and four to put him two-under on the day and overall.
Shane Lowry, partnering Thornton, had a bogey on the first, but picked up three birdies by the eighth to leave him two under for the day.
Thornton opened with three consecutive bogeys to leave him three over.
A massive crowd had again turned up at Ballyliffin to watch McIlroy, the world number eight, opened with three pars.
McDowell was to finish on par for the tournament, his only blemish in his final round coming on the 18th hole where he bogeyed. Two birdies earlier saw him finish one under for the day and level for the event.
A double bogey on the par three fifth saw McIlroy move to two under, but three consecutive biridies on 11, 12 and 13, saw him ove to two-under, only to bogey the 15th and return to one under.
The Holywood man, who was hosting the event via The Rory Foundation, for the fourth successive and final year, parred the last three holes to go into the clubhouse at joint 28th on two over for the week.
Leader Van Rooyen picked up a birdie on the fourth, but nback to back bogeys on six and seven saw his lead reduced to two overall, with Spaniard Jorge Campillo charging up the leaderboard with four birdies and an eagle on the front nine taking him to the turn at six under for the day, 11-under overall.
It was beginning to get tight at the top of the leaderboard with Campillo getting another birdie, his first on the back nine.
He moved to 12-under, while overnight leader dropped to 12-over as his front nine finished with two over at the turn.
Scotland' Russell Knox made it four sharing first place as he climbed the leaderboard with a four under by the 12th.
A birdie on 11 brought New Zealander Ryan Fox onto two-under for the day and a share of the lead.
Birdies for Campiollo, Knox and Fox saw them share the lead at 13 under as the leaders came into the last four holes.
Jon Rahm made it three birdies in succession on the 17th to improve to 11 under, but time was running out for the world number five.
Rahm, who had driven out of bounds on the second hole on his way to a triple-bogey seven, had further reason to wonder what might have been after almost making an eagle on the 18th after his approach clattered into the pin.
The resulting tap-in birdie completed a remarkable 66 and left him one shot behind clubhouse leader Campillo, who had saved par on the same hole to complete a brilliant 65.
Fox missed from 10 feet for a potentially decisive eagle on the 17th, but the tap-in birdie saw the Kiwi reclaim the outright lead on 14 under par.
However, just as he walked towards his ball on the 18th fairway, up ahead on the green Knox holed from 40 feet across the green for a birdie to join him in a tie for the lead.
Fox then had a birdie opportunity chance on 18 to go into a one-shot clubhouse lead, but missed the 10-foot effort to leave him and Knox both on 14-under overall and a play-off to come with one group still to play 18.
That left Van Rooyen needing to play the final two holes in two under par to make it a three-way play-off - the overnight leader parred both leaving Knox and Fox to contest a sudden-death play-off on the 18th, with a first prize of Â£877,000 on the line and, in Knox's case, a huge number of points in Ryder Cup qualifying.
It looked like advantage Fox as he hit another superb drive on the first play-off hole and pitched to around the same distance from which he missed on the 72nd hole.
But Knox also saw his approach from the bank of a fairway bunker end up remarkably close to where he holed from in regulation and amazingly again drained the long birdie putt.
Fox then saw his attempt to keep the play-off alive catch the edge of the hole and spin out, giving Knox an incredible victory.