The Honda Racing rider claimed a maximum 25 points and valuable podium credits after he was declared the winner of Saturday’s opening race at the Northamptonshire circuit.
Irwin was holding third place when McAMS Yamaha riders Tarran Mackenzie and Jason O’Halloran crashed out on the penultimate lap.
The race was stopped, with Irwin taking the win ahead of VisionTrack Ducati’s Christian Iddon.
With a 32-point buffer inside the Showdown top-eight going into yesterday’s final two races, Irwin was all but guaranteed his place among the title fighters barring any major mistakes.
He finished 13th and ninth in races two and three, leaving the Carrickfergus man in seventh in the Showdown standings ahead of the final three rounds, which get under way from September 24-26 at Oulton Park.
“A place in the Showdown was the goal coming into this weekend, I didn’t think we would get a race win with the form of Yamaha and Suzuki from the past two seasons here, so to tick that off in race one was amazing,” said Irwin.
“In race two we tried something with the bike and today was a day we could slightly get away with it with the points buffer, so it wasn’t too detrimental to us, but it set us up for a nervy last race!
“Today has been an enjoyable day, the Fireblade in race three was a big step from the bike we had in race two set-up wise. To make the Showdown, and go into it with some podium credits – considering we had a difficult start to the season technically, and with an injury – it hadn’t been the smoothest of starts this year, but we made it into the Showdown.”
His younger brother Andrew was ruled out of Showdown contention after finishing 11th in Saturday’s race, although team-mate Danny Buchan fought back from a crash on Saturday to finish 12th and fourth on Sunday, ensuring the SYNETIQ BMW team made it through to the Showdown.
The final eight riders who will battle it out for the title are: Jason O’Halloran (McAMS Yamaha), Tarran Mackenzie (McAMS Yamaha), Christian Iddon (VisionTrack Ducati), Tommy Bridewell (Oxford Products Racing Ducati), Peter Hickman (FHO Racing BMW), Danny Buchan (SYNETIQ BMW), Glenn Irwin (Honda Racing) and reigning champion Josh Brookes (VisionTrack Ducati).