PJ Banville proved Waterford’s super-sub with a stoppage time winner which ended Rockmount's FAI Cup dreams.
Willie Bruton, who had rescued Waterford in the first game with a late equaliser, gave United the perfect start with a goal inside the first minute.
David Breen released Daryl Murphy down the right flank to cross for Bruton to head home.
Rockmount leveled affairs on 77 minutes when Pat Keane set up defender John Busteed to volley home.
Waterford substitute Vinny Sullivan was then sent off for an incident following the goal celebrations.
Ken Coleman set up Banville to shoot home on 92 minutes to put the Blues into the semi-finals after a thrilling replay at Turner’s Cross.
Filed by Barry McEneaney
