Harps started well and had a couple of decent chances through McHugh and Shaun McGowan.
But it was Limerick who drew first blood when highly-rated striker Denis Behan beat Ciaran Gallagher on the half hour.
Harps kept to score to a minimum as the sides went into the break.
However disaster struck after the restart when McGowan got his marching orders and form the resulting penalty was slotted home by Shane Treacy to make it 2-0.
The game was over as a contest by now and Pat Scully’s side made it 3-0 through Gaffney.