The Carrickfergus man paid tribute to PBM BeerMonster Ducati team boss Paul Bird afterwards, who passed away two weeks ago aged 56 following a short illness.
Irwin, sporting a special edition Shoei helmet in honour of Bird’s successes over the years as the owner of the renowned Paul Bird Motorsport Team, was only headed briefly on the 12th and final lap by team-mate and championship leader Tommy Bridewell, who gained better drive out of the first corner before Irwin battled back on the run to Island Bend.
A mistake on the brakes at Hizzy’s then cost Bridewell any hope of challenging for the win, with the Wiltshire man dropping back and finishing fourth.
Irwin won by four-tenths-of-a-second from Lee Jackson (Cheshire Mouldings Kawasaki), with Leon Haslam rounding out the top three on the ROKiT BMW.
Josh Brookes (FHO Racing BMW) and Kyle Ryde (OMG Yamaha) completed the top six.
“We’re trying to take the emotion out of it this weekend and it’s really difficult, but that helps,” said Irwin.
“That is exactly what Paul Bird would have wanted – two team-mates fighting for the win.
“I’m just really enjoying this right now, it’s been an amazing season so far and let’s keep it going.”
Davey Todd, deputising for the injured Danny Buchan, was 12th in his first race on the SYNETIQ BMW. Todd has signed a deal to ride for TAS Racing on a Milwaukee BMW in 2024 after parting company from Clive Padgett’s Honda team.
Ballymoney’s Michael Dunlop was 19th on the Crendon Honda by Hawk Racing as the 25-time Isle of Man TT winner fill in for the injured Josh Owens.
Honda Racing’s Andrew Irwin remains sidelined after a crash in testing at Knockhill at the end of May, but the Carrickfergus man is nearing a return.
In the opening National Superstock 1000 race, Donegal’s Richard Kerr finished second on the AMD Motorsport Honda behind Dan Linfoot, with Alastair Seeley third on the SYNETIQ BMW in a close race, with the trio separated by under half-a-second.
On Sunday, the two 18-lap British Superbike races are scheduled for 13:15 BST and 16:15 BST.