Oran Kearney’s men opened the scoring on 20 minutes when Jamie Glackin played the ball across to the right flank for Andy Scott, whose cross was met by a bullet header from McGonigle which found the bottom left-hand corner.
As half-time approached Aaron McCarey produced a superb reaction save to stop Matthew Shevlin’s strike with McGonigle sending the rebound well wide.
The first half drama wasn’t done there though as deep into stoppage time, Glackin’s powerful effort looked like it was going to double the away side’s lead.
Glentoran stopper McCarey made an impressive outstretched save to palm the strike away, however, as he attempted to gather the danger, he clashed with Luke McCullough and fell.
The alert Lyndon Kane was then able to lift the ball over the Glentoran stopper and into the back of the net for his first goal of the season.
Glentoran started the second period with intent with Graham Kelly having to make a crucial clearance off the line to stop David Fisher’s strike, before Rory Brown was forced to make a diving save to stop Junior’s header.
The Bannsiders further extended their advantage in the 70th minute when Glackin backheeled McGonigle’s corner back to the January signing and his resulting strike from a tight angle went through the legs of McCarey and into the net.
Things went from bad to worse for the home side with just five minutes remaining when Fuad Sule received his second yellow card of the game for a foul on Shevlin after being cautioned for a challenge on Stephen O’Donnell just nine seconds into the game.