Irwin finished eighth in Saturday’s opening race at the Cheshire circuit and crashed out of race two on Sunday, hurting the shoulder he injured in a crash at Donington Park.
He set the second fastest time in the race before the incident, giving the Carrickfergus man a front-row start for race three.
However, Irwin could only finish seventh and the 31-year-old is now 64 points behind new championship leader Tarran Mackenzie.
“I don’t know how we’re in the Showdown because it has been a mad year,” said Irwin.
“I knocked my shoulder there a little bit and I didn’t realise until I went to put my leathers on.
“The race was enjoyable even though we crashed out but we really struggle for pace on the Fireblade here, but we had some pace in that race and I ended up second on the grid.
“We’ll just try and hang in there and see where we need to improve when we come back here next year because it’s more about the bigger picture.”
The first of three title-deciding Showdown rounds produced plenty of drama, with Jason O’Halloran losing his championship lead to McAMS Yamaha team-mate Mackenzie following two crashes. The Australian slid off in Saturday’s first race and ended the weekend on a low note with another crash in race three.
Mackenzie won race one from VisionTrack Ducati riders Josh Brookes and Christian Iddon, but it was Tommy Bridewell – sixth on Saturday – who forced his way into the reckoning after a double on Sunday on the Oxford Products Racing Ducati.
Bridewell beat O’Halloran and Mackenzie by half-a-second in race two before dominating the final race, taking victory by almost six seconds from a rejuvenated Brookes, who was a faller in race two. Bradley Ray completed the rostrum on the Rich Energy BMW ahead of FHO Racing BMW’s Peter Hickman and Mackenzie.
Going into the penultimate round this weekend at Donington Park, Mackenzie leads the way by only two points from O’Halloran, with Bridewell seven points back in third.
Andrew Irwin was 17th in race one after running off the track with the SYNETIQ BMW rider finishing ninth and eighth in Sunday's races.
Ballymoney’s Michael Dunlop claimed a points-scoring finish in 14th place in race two.
Dunlop was filling in for the injured Danny Kent in the Buildbase Suzuki team. The 32-year-old was 19th in race one and 17th in race three.
In the British Supersport class, Lee Johnston finished fourth on the Ashcourt Racing Yamaha in Saturday’s Sprint race. Dublin’s Jack Kennedy emerged victorious and wrapped up a Sunday yesterday, when Ben Currie – who crossed the line first – received a three-second long lap equivalent penalty for overtaking before the green flag was shown after the Safety Car was brought onto the circuit.
Johnston was demoted to eighth place after he also received a penalty for 'overtaking under the Safety Car'.
Young gun Scott Swann was promoted to third place as he celebrated his maiden podium in the class on the IFS Team Swann Yamaha. Swann also qualified on pole after mastering the tricky conditions on Saturday.
Former champion Kennedy now leads the championship by 13 points from Currie, with Johnston 25 points further back in third.
Rising prospect Cameron Dawson won the Junior Supersport race to increase his title lead, while Eugene McManus twice finished on the rostrum in third in the Junior Superstock races.
Eglinton’s David Allingham was 14th and 13th in the Superstock 1000 class.