In scorching conditions around the 8.9-mile ‘Triangle’ course, Irwin and Milwaukee BMW’s Davey Todd engaged in a repeat of their battle in Thursday’s opening race, which went down to the sixth and final lap.
With nothing between them, Todd was lining up a pass as they approached Juniper chicane but a slower rider skittled his tactics, preventing the 28-year-old from executing his plan.
That left Hager PBM Ducati’s Irwin to clinch an historic win by 0.382s for a double following his victory in Thursday’s race.
Michael Dunlop finished third on his Hawk Racing Honda, 19 seconds down, despite incurring a 10-second time penalty after failing to stop when he ran on at Magherabuoy chicane on the first lap.
Irwin wrapped up his 10th Superbike victory in a row and his seventh on Ducati machinery to take over as the most successful rider ever in the blue riband class from Joey Dunlop and Michael Rutter.