Wales international Jonny Williams broke the deadlock when he converted home after being found by McKirdy, but the lead didn't last long thanks to a stunning strike by young loanee Kian Harratt in his first start for the club.
On the stroke of half time, the Robins went back in front after goalkeeper Lucas Covolan was sent off following a collision with McKirdy, with the resulting penalty kick being fired home by Ben Gladwin.
This sending off seemed to knock the stuffing out of Vale and the icing was put on the cake inside 69 minutes when McKirdy's cross took a nick off Nathan Smith and found its way past Aidan Stone, with it being put down as an own goal, while still capping off a memorable day for the forward who featured 14 times for the Valiants last season.
This result sees Darrell Clarke's side fall out of the play-offs and drop down to ninth, while Swindon makes the jump to fourth, only a point adrift of the automatic promotion spots.
Story of the game
In what was a lively start to the affair with both teams having come out strong, it was the Robins who had the first opportunity as McKirdy was close to making it a dream start to his return to Vale Park.
Covolan red card ultimate killer for Vale
Despite being second best in the first period, a great strike by Harratt had got Vale back in the game and there is no doubt that the three points would have still been well on the cards.
This is especially considering that Clarke would have had the option to make some adjustments at the break with him having the likes of Bolton Wanderers loan winger Dennis Politic and Leeds United loanee Ryan Edmondson at his disposal.
However, the possibility of this was taken out of the window when Covolan received his marching orders following an off the ball incident with McKirdy- a moment which really made it an uphill battle for the Valiants.
To make matters worse, even though Stone got close to keeping it out, Gladwin managed to convert the penalty leaving Vale a man and goal down with 45 minutes on the pitch.
In the second period with James Wilson being forced to come off due to the dismissal, leaving the Valiants a forward short, they really struggled to get effective service into Harratt and the Robins with the man advantage were able to dominate the midfield battle.
The third goal really ensured that maximum points would be going to Garner's side and towards the end of the game the hosts just seemed content with making sure that the score didn't end up becoming embarrassing as with a man less, even if they pushed men forward, a route back into the game was always going to be unlikely.
Star Man- Harry McKirdy
During his time at Vale Park, McKirdy was out of favour with him only making two starts in the league before leaving the club to join Swindon ahead of the season.
With his performance today, he certainly showed the ability that he does possess though as the 24-year old looked a threat every time he got on the ball and wasn't afraid to drive at the hosts back three.
The forward also had a play in his side's all three goals and was instrumental to everything they did in the attacking phase of play-a big part tio why they came away with all three points.
Despite not quite being able to hit it off during his spell at the Valiants, this season it is clear that at his new club Garner is getting the best out of him with him having netted 12 goals and accumulating six assists in all competitions.