Irwin and his partner Laura, who have a son, Freddie, welcomed Gia Annie Irwin into the world last Saturday.
The Honda Racing rider is nursing a shoulder injury, which he aggravated in a crash earlier this season at Donington Park, and is due to undergo surgery in the coming days.
Irwin was ninth in Saturday’s opening race and ninth again in race two, before pulling out of the final race on Sunday with an engine problem.
The 31-year-old finished the season in eighth place in the Showdown, with Scotsman Tarran Mackenzie clinching the title in style with a dominant hat-trick of wins on the McAMS Yamaha.
Irwin said: “I’m enjoying the bike and the last few rounds I’ve had very little pressure and we’ve been trying things with the bike.
“So I’ve been just getting stuck in, though it’s difficult with my shoulder. I’m riding different because of that and using my legs more, and trailing the brake to make it turn into left corners a bit more.
“I haven’t had any sleep because my little baby daughter was born last Saturday night, so I’ve a special had a special helmet done and big thanks to Rage Designs for that.
“I’m just high on adrenaline so that’s why I’ve been going well!
"It's been a rollercoaster of a season and huge thanks to my team, we've come a long way this year from where we started and they turned it around," he added.
"We made the Showdown, four podiums, a race win and a couple of pole ositions in what I would call a difficult year - it could have been much worse."
His brother Andrew finished sixth in Saturday’s race and crashed out of race two on Sunday on the SYNETIQ BMW. The Carrick man finished the final race in ninth place.
Mackenzie, who lifted the title 25 years after his father Niall famously edged out his Cadbury Boost Yamaha team-mate Jamie Whitham in 1996, was in imperious form as he was crowned British champion for the first time.
The 25-year, now touted for a move to World Superbikes in 2022, said: “Honestly, I don’t know what to say. I’ve got so many people to thank so I’m not going to attempt it, but I can’t believe it.
“It’s been a roller-coaster of a year. It’s nice to have all the fans back, especially at Brands Hatch so I thank everyone that’s come out to support me over the years.
“And to do it 25 years since dad won his title is amazing – it shows how old he is! I can’t thank everyone enough.”
Mackenzie won Saturday’s race by 0.160s from Tommy Bridewell (Oxford Products Ducati), with Christian Iddon third on the VisionTrack Ducati.
He made it a double on Sunday, snatching victory on a thrilling final lap, timing his move to perfection as he went underneath Bridewell at Clearways and holding on to win by 0.087s. Mackenzie’s team-mate, Jason O’Halloran, finished third ahead of Iddon.
The victory was enough to give Mackenzie the title with one race to spare.
In the final encounter, Mackenzie and Bridewell again battled it out, with the newly crowned champion prevailing once more on the last lap after Bridewell ran wide at Clearways following a last-gasp move.
O’Halloran finished on the podium in third ahead of outgoing champion Josh Brookes (VisionTrack Ducati).
Mackenzie’s final advantage in the Showdown was 36 points over Bridewell, with O’Halloran third, four points further back.
2021 BSB Showdown: 1 T Mackenzie (Yamaha) 1202 points, 2 T Bridewell (Ducati) 1166; 3 J O’Halloran (Yamaha) 1162; 4 C Iddon (Ducati) 1141; 5 P Hickman (BMW) 1092; 6 J Brookes (Ducati) 1079; 7 D Buchan (BMW) 1075; 8 G Irwin (Honda) 1055.