Their last Easter meeting before Wigan move out of Central Park seemed to have slipped through Saints' fingers when a Wigan team who had been under the cosh virtually throughout fashioned a try to take them into the lead with less than 10 minutes to play.
Greg Florimo hoisted a high kick, Chris Smith made the fundamental mistake of allowing the ball to bounce and Danny Moore stretched to claim it and score. When Andy Farrell landed a difficult conversion to give Wigan a two-point advantage it looked as though all Saints' domination had been in vain.
However, Ellery Hanley's side showed him, on his own return to the ground where he achieved so much, that they can summon up the tenacity and determination that characterised his playing career.
With five minutes left, Keiron Cunningham made a probing run to the right, but there was still nothing on offer for Iro, his head swathed with bandages, when he took the ball. The Kiwi, so often a big-match winner for Wigan, shrugged off two tacklers and kept a third at bay as he plunged for the line. It was a try and a result that Saints had earned.
They had gone behind after 10 minutes when Florimo swatted away an ineffectual challenge from Paul Sculthorpe to go over, but they set the pace for the rest of the first half.
Saints battered the Wigan line without being able to break through and the home side even extended their lead against the run of play with a penalty from Farrell.
When Anthony Sullivan narrowly failed to squeeze in at the corner from Paul Newlove's pass it seemed as though Wigan might have succeeded in weathering the storm but, immediately before half-time, the same combination set up the crucial score.
There was an element of luck about it, Tommy Martyn's kick rebounding into his arms and Newlove taking advantage of the overlap to give Sullivan the room he needed to mark his return to the side with a try.
Straight after the break Saints struck again, with two more of their former Wigan players responsible. Sean Long, whose departure as a make- weight in a transfer deal now looks one of Wigan's less inspired pieces of business, put up a high kick and Paul Atcheson, discarded after just one season at Central Park, leapt high to catch and score.
Martyn's conversion gave Saints a four-point lead that they held, thanks to their own determined defence and Wigan's tendency to drop the ball in promising positions.
Hanley's perming of his depleted forward resources by using Fereti Tuilagi in the second row for most of the game was an unqualified success, and the youngsters, Mark Edmundson and Tim Jonkers, also made impressive contributions. So there was every reason for heads to drop after Moore's try threatened to rob them of their reward.
Even after Iro's winner, Wigan thought they should have had an equalising penalty when Mick Cassidy had the ball ripped from his grasp.
"A couple of tight decisions went against us," said the Wigan coach, John Monie. "But we were ahead with eight minutes to go and we should have wrapped the game up. They [St Helens] showed a lot more character; it was more like the Wigan-St Helens games from the past.
"Ellery is a very single-minded person, with a very strong self-belief and he has made St Helens pretty hard to crack. They have a much better defence than last year."
Hanley also had praise for his players: "I never thought the chance had gone. If there's time on the clock, there's always a chance. It was a big effort from the whole squad. We showed a lot of character."
Wigan: Radlinski; Robinson, Connolly, Moore, P Johnson; Florimo, Reber; Cowie, Cassidy, O'Connor, Haughton, Gilmour, Farrell. Substitutes used: Mestrov, West, Ball, Goldspink.
St Helens: Atcheson; Tuilagi, Iro, Newlove, Sullivan; Martyn, Long; O'Neill, Cunningham, Nickle, Perelini, Edmundson, Sculthorpe. Substitutes used: Stewart, Smith, Jonkers.
Referee: S Cummings (Widnes).