The visitors made a strong start to the second half and were rewarded with a try to powerful winger Joe Robwata in the 36th minute of the 70-minute clash.
Ireland completed his hat-trick in the 46th minute after a clever break from his second five-eighth Kere Penetito. McLeod added the extras.
Outstanding second five-eighth Hausia Vea scored his first try for the visitors after a solo run in the 51st minute. Centre Mahaki-Jay Hetet-Wairau converted.
Any hope the visitors had of a late comeback was dashed when McLeod put the hosts 29-12 ahead with a 55th minute penalty. Vea scored his second try in the 62nd minute.
This was a first round fixture in the Central North Island competition and one aspect of play the Hastings team will need to improve if they want to be a force in the competition this season is their penalty count. They conceded 12 penalties, six more than the Hamilton side.
"I'm so proud of the team. You could see the heart and passion in the boys," elated St John's College Hastings captain and lock Ben Gardiner said afterwards.
"We've been training a lot and it's good to see the boys finishing things off," added the son of former Magpies lock Alan Gardiner.
The visitors captain and hooker Kees Mortleman admitted his side didn't execute as well as they should have and didn't support their ball carriers enough.
All of the St John's Hastings players deserve praise for their efforts. McLeod, Ireland and fullback Ryan Lindeque were classy in the winning backline.
Gardiner, No 8 Caleb Burke and prop Caden Mouat-Harvey were prominent in a dominant host pack.
Scorers:
St John's College (Hastings) 29 (Max Ireland 3, Emanuelu Lemuelu tries; Mitchell McLeod 3 cons, pen), St John's College (Hamilton) 17 (Hausia Vea 2, Joe Robwata tries; Mahaki-Jay Hetet-Wairau con). HT: 19-0.