The PBM Be Wiser Ducati rider was in red-hot form as he beat fellow British Superbike contender Christian Iddon (Tyco BMW) in the feature race of the day, which was re-started over six laps following a red flag.
Essex man Danny Buchan, who was bidding for a third straight Sunflower victory, finished in third place on the MSS Kawasaki.
Irwin and Iddon – making his Sunflower bow – had been locked in a furious battle for the lead when the red flags came out in the initial race. A shortened six-lap re-run was announced and although Iddon hit the front on lap one, Irwin quickly took over the lead at the start of the second lap and led all the way to the flag.
The Ulster rider added his name to the illustrious list of previous winners, which includes his father Alan – a winner in 1982 and 1986 – after taking the win by three tenths of a second from Iddon, with Buchan 1.3 seconds behind as he rounded out the top three.
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Nikki Coates took fourth on the KP Kawasaki ahead of British Supersport frontrunner Andrew Irwin (Gearlink Kawasaki), who was the first 600cc rider home in fifth. Masters Superbike champion Robert English completed the top six on the IFS Yamaha R1.
Nico Mawhinney, who qualified fourth fastest on the front row, dramatically crashed out on the warm-up lap.
Irwin opened his account earlier in the day with a convincing win in the first Superbike race from Iddon and Buchan. The 26-year-old then put the icing on the cake as he wrapped up a terrific treble later in the afternoon, beating Buchan as Robert English finished in third.
Essex rider Christian Iddon missed the final Superbike race after leaving the circuit following the Sunflower event to catch a plane home.
In the first Supersport race, Gearlink Kawasaki’s Andrew Irwin edged a thrilling race by 0.189 seconds from David Allingham (EHA Racing Yamaha) with Carl Phillips taking a close third.