The Ballybofey side netted twice before the break to remain in control despite a late surge by the home side.
But it was far from a straight forward win for Harps.
They first had to watch as their goalkeeper Ciaran Gallagher lay motionless on the ground in the 20th minute after a clash with United’s Seamus Long.
Gallagher was eventually taken to hospital but it is unclear as to the extent of his injury.
Gallagher was replaced in nets by sixteen year old Shaun Patton who more than lived up to the big occasion.
Gallagher’s loss seemed to spur Harps into action with Simon Treacy having a hand in both Harps goals before the break.
Treacy poked home the ball from close range for the opener and then he watched as his corner was deflected into the Waterford net by Seamus Long.
Waterford dug deep despite being two down and were awarded a penalty when Peter Hutton was penalised for a foul inside the area.
But young goalkeeper Patton lived up to a dream debut and managed to save the effort from Gary Dempsey with an outstretched foot.
United continued to press but Hutton managed to organise his defence and kept the threat to a minimum of decent chances.