Portmarnock 1 Longford Town 2
Longford Town came from behind to book their FAI Cup semi-final place against Waterford United.
Juniors Portmarnock shocked the Premier Division side by taking a 32nd minute lead.
Keith Bruen was credited with the goal as his ball into the area caused confusion between Stephen O'Brien and his defence to end up in the net.
Town rallied well and Keith O'Connor headed them level from Sean Prunty's cross within two minutes of the restart.
Portmarnock then paid the penalty when O'Connor was taken down on 82 minutes and Paul McNally shot home from the spot.