The striker had silenced the home crowd by firing home against his former club, putting their play-off spot in jeopardy.
But that extra momentum proved enough, as Lee Gregory, Saido Berahino and Jordan Storey all struck before the break.
George Byers then nodded home in the closing stages to ensure the Owls would finish in League One’s top six.
Danny Cowley stuck with the same side that fought from behind to beat Wigan earlier in the week, although teenage defender Liam Vincent was elevated to the bench.
Byers started for the home side, while fellow former Fratton loanee Nathaniel Mendez-Laing was named among their substitutes.
The visitors got off to a flying start and broke the deadlock in just the fourth minute to stun their opponents.
HIRST – who began his career with the Owls – had been roundly booed by the home support, but he was the man who got the goal.
The striker slotted the ball home after Ronan Curtis had raced onto a Connor Ogilvie pass and delivered the ball into the mixer.
But Wednesday soon picked things up and Gavin Bazunu had to be alert to deal with an under-hit Clark Robertson back-pass before Berahino could capitalise.
The keeper could do nothing to prevent the equaliser, however, as Gregory neatly diverted Marvin Johnson’s pass into the bottom corner.
And they kept pushing forward, with Bazunu beating away an angled Gregory strike and then acrobatically turning Barry Bannan’s effort over the bar.
The hosts eventually made their pressure count, though, and Berahino forced the ball home after a corner had bounced dangerously around the box.
It soon got even worse for the Blues, as Byers cut in from the right and squared for Storey to convert before the break.
Sheffield Wednesday 3
The second half did not begin in the same explosive manner as the first, although a series of fouls gave both sides free kicks in dangerous areas.
One was earned by Curtis on the left, but the delivery from Michael Jacobs was flicked onto the roof of the net by Marcus Harness.
Hirst was then quickly surrounded by a sea of defenders after good link-up play between Harness and Hayden Carter had provided an opening for the striker.
Bazunu came to the rescue at the other end, denying Byers after he had been picked out inside the area by Johnson.
There were certainly not an abundance of clear-cut openings to excite the large crowd, though, as the contest began to drift along.
Cowley tried to liven things up with a substitution and Jacobs made way for Tuesday night’s goal hero Aiden O’Brien.
It almost had the desired impact, with the forward flicking the ball against the outside of the post following great work on the left by Curtis.
But the Owls then almost scored themselves, with Gregory lifting a shot over Bazunu and onto the roof of the net.
It was certainly livening up and when Sean Raggett met a Harness corner, his header was tipped against the crossbar by Bailey Peacock-Farrell.
Pompey were certainly not giving up the chase and it took another fine stop from the home keeper to keep out a fierce Hirst drive on 79 minutes.
But Bazunu had to show his athleticism once more to turn a fierce drive from substitute Callum Paterson over the bar.
Sheffield Wednesday did wrap things up from the resulting corner, however, as Byers nodded home from a Josh Windass delivery.
Jay Mingi came off the bench and had a chance to grab a stoppage-time consolation that would have seen the Blues finish ninth instead of 10th, but could not keep his shot down from a Hirst cut-back.
Sheffield Wednesday (3-5-2): Peacock-Farrell; Storey, Hutchinson, Palmer; Hunt, Byers, Luongo, Bannan (c) (Paterson 83), Johnson; Berahino (Windass 76), Gregory (Dele-Bashiru 88)
Goals: Gregory 17, Berahino 36, Storey 40, Byers 86
Booked: Gregoy, Paterson, Byers
Subs not used: Wildsmith, Gibson, Dunkley, Mendez-Laing
Pompey (4-2-3-1): Bazunu; Carter, Raggett, Robertson (c), Ogilvie; Morrell, Thompson (Mingi 88); Harness, Jacobs (O’Brien 64), Curtis; Hirst
Goals: Hirst 6
Subs not used: Webber, Romeo, Vincent, Williams, Walker
Referee: Anthony Backhouse
Attendance: 33,394 (2,558 Pompey fans)