Substitute Deirbhaile Beirne nodded home deep in injury time as Peamount United denied DLR Waves what would have been a famous win at the UCD Bowl.
The winger sent a looping header over Waves keeper Eve Badana, who had earlier saved Steph Roche's penalty, to crush the home side’s resistance.
Katie Malone had put Waves in front with a well-taken header early in the second half and Peas looked set to be frustrated by a stubborn defensive effort.
Beirne, who spent a year and a half with Waves in their previous guise of UCD Waves before joining Peamount in 2018.
And she returned to haunt her former teammates with a brilliantly-taken finish shortly after the allotted four added minutes had passed.
It was cruel fortune for Ireland keeper Badana, who had pulled off a number of key stops, not least correctly diving to her right to palm away Roche’s penalty.
The game had been relatively even to that point as Sadhbh Doyle went close for Peas early doors before Kate Mooney tested Niamh Reid-Burke at the end of a flowing move.
Louise Corrigan, making her first Waves start following a cruciate knee ligament injury sustained as a Peas player, clipped Áine O'Gorman’s heels as the captain looked to dig out a cross.
Roche took the honours but, just as she had for Tiegan Ruddy’s spot-kick when the sides faced at the end of last season, Badana read the striker’s intention and parried clear.
Peas then saw efforts from Erin McLaughlin and Roche cleared off the line as they went into the break the stronger side.
It was Waves who took the lead just five minutes in, however, a clever outside pass from Nadine Clare putting Sarah McKevitt in space.
The winger battled past Tara O’Hanlon and clipped an inch-perfect cross to the back post, where Malone calmly nodded home.
Badana delivered a goalkeeping masterclass in the final 40 minutes, saving brilliantly from McLaughlin, Doyle, O’Gorman and Lauryn O’Callaghan as Peas piled on the pressure.
Their endeavour looked set to be in vain as Waves spent most of the final minutes in the visitors’ half.
But they continued to the last and, when O’Gorman crossed deep, Beirne peeled away to steer a terrific header across goal that looped over the keeper and in.
DLR Waves: Eve Badana; Lynn Craven, Louise Corrigan (Oleta Griffin 62), Jess Gleeson ©, Aoife Brophy; Nadine Clare (Fiona Donnelly 76), Mia Dodd, Kate Mooney; Sarah McKevitt (Rachel Doyle 69), Katie Malone (Sophie Watters 69), Carla McManus (Avril Brierley 76).
Peamount United: Niamh Reid-Burke; Lauryn O’Callaghan, Karen Duggan, Chloe Moloney (Deirbhaile Beirne 84), Tara O’Hanlon; Alannah McEvoy, Dora Gorman, Sadhbh Doyle, Erin McLaughlin; Áine O’Gorman, Stephanie Roche (Michelle Doonan 70).