A try from Simon Zebo and 11 points from the boot of Ronan O'Gara seemingly set up a grandstand finish for Munster in the second half.
But stirling work from the Ulster defence - led by man-of-the-match Stephen Ferris - ultimately saw them home.
Thanks largely to the boot of Pienaar - plus a wonderful individual Craig Gilroy score and an Ian Humphreys drop goal - Ulster were 19 points to the good just after the half-hour mark.
A shellshocked Munster side got themselves back into it with a converted score from Zebo after 34 minutes and an O'Gara penalty on the stroke of half time.
Two more penalties, either side of another Pienaar effort, from O'Gara looked to have set the stage for one of Munster famous late wins.
But despite the last quarter being dominated by the home side it was Pienaar and co that held strong to take the win.
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