His clumsy challenge brought down the Iron full-back Williams in the game's decisive moment which left his manager Darren Ferguson shaking his head in disappointment.
Earlier there had been little to test either goalkeeper in a lacklustre first half from both teams.
Peterborough did have a golden opportunity to get themselves back on level terms just five minutes into the second half with a good link-up between their normally proficient front pair.
Aaron McLean got behind the home defence and squared an inviting ball low into the box for fellow frontman Craig Mackail-Smith, who somehow failed to apply the finishing touch after Iriekpen seemed to miss his clearance..
Peterborough continued to have plenty of possession but struggled to break down a Scunthorpe defence in which teenager Krystian Pearce had a good debut on loan from Birmingham.
In the closing stages it was the home side who missed the chance to finish comfortable winners.
First Gary Hooper clipped the ball over his marker but lifted the ball over the bar and then Garry Thompson fired awfully wide with only keeper Joe Lewis left to beat.
Substitute Ian Morris also went close with a shot that Lewis did well to turn behind and Jonothan Forte had a shot blocked as United finished in the ascendancy.