The Nelson bowler beat Scott 25-20 in last year's final to join a select group of only five bowlers who have won two or more titles in the event.
McIlroy (22), a member of the New Zealand Commonwealth Games squad, used powerful drives to smash out Scott's shot bowls and win 25-18.
He displayed this aggression on the seventh end to send the jack into the ditch to gain one shot when down on the head.
It was the feature game of the opening day at the North East Valley green in a tournament that features the best bowlers in the country.
Promising North East Valley bowler Mark Watt (20), a member of the New Zealand Commonwealth Games team, started impressively with a 25-17 win over Dave Archer (Dunedin) in his opening game.
Doug Thomas (North East Valley), who has battled leukaemia for 11 years and had a stem cell and bone marrow transplant in Christchurch five years ago, started impressively with a 25-20 win over Steve Posa (Waikato).
But he met his match against new international Shayne Sincock (Canterbury) in the second round and was beaten 20-25.
Sincock caused a major upset in the first round when he beat double World Bowls gold medallist Gary Lawson (Canterbury) 25-19.
Lawson, who was beaten by Posa 17-25 in the second round, ended the first day without a win.
It was a topsy-turvy day and Sincock (44), a courier driver, was one of only six bowlers who won both their games.
The others to win two games were Nigel Wright (Dunedin), Dan Delany (Auckland), Glen McDonald (Canterbury), Bill Clements (Alexandra) and Jamie Hill (Auckland).
Wright beat Lance Pascoe (West Coast) 25-13 and then caused a major upset by beating McIlroy 25-19.
Delany had a big 25-12 win over former New Zealand representative Justin Goodwin (North Harbour) and had the edge when he beat Watt 25-20.
McDonald, a former international, outclassed Wayne Hardcastle (North East Valley) 25-8 in the first round and won a close game against New Zealand singles champion Richard Collett (Nelson) 25-22.
Collett (29), a member of the Commonwealth Games squad, was also beaten by Chris Le Lievre (Auckland) 14-25 and ended the day without a win.
Clements, who has won 52 Otago Central titles, beat former World Bowls representative Paul Girdler (Dunedin) 25-20 and Hardcastle 25-19.
Hill, a member of the Commonwealth Games squad, beat Mike Cranstoun (North East Valley) 25-18 and former international Jim Scott (North East Valley) 25-10.
Five more rounds of section play will be held over the next two days and the quarterfinals will begin at the North East valley green tomorrow afternoon at 2.45pm.
