One personnel change from the playoff-winning first XV saw Declan Fitzpatrick drop to the bench to make way for John Afoa at tighthead, with the Kiwi fully recovered from the hamstring strain which kept him out of the 27 - 18 defeat of the Scarlets.
Leinster set the pace early on, Shane Jennings grabbing a third-minute try after a strong leap from skipper Leo Cullen at the lineout. Jonathan Sexton converted without difficulty, and with Ulster conceding several penalties in quick succession from the restart, the Racing Metro-bound outhalf added three more points on the eight-minute mark.
Then a smart charge-down from Paddy Jackson as Isa Nacewa attempted to clear gave Ulster their first chance of the encounter, and, although Isaac Boss beat his opposite number Pienaar to touch down behind his own line, the resulting five-metre scrum - while eventually coming to nothing - served as a clear indication of the Ulstermen's superiority at scrum-time.
Indeed, Ulster should have scored on 17 minutes, Robbie Diack inches away from touching down as the Leinster backs held him up after a scintillating burst through the middle by Tommy Bowe, ably carried on by Jackson. Little time was needed to get back into Leinster territory, however, and this time, with Cian Healy failing to release Nick Williams from his tackle in good time, Pienaar kicked over an elementary three points on 24 minutes to get his side on the scoresheet.
With Ulster enjoying the majority of possession as the half-hour mark approached, a rare missed penalty from Sexton ensured the gap remained only at a converted try. However the Leinster out-half made amends moments later with a 45-metre kick, which was rubbed out with five minutes of the half to go thanks to a sublime penalty into the strong wind from Pienaar.
Sexton closed out the period with this third successful penalty of the afternoon, sending Ulster off at the break 10 points adrift.
Half-Time Score Ulster 6 Leinster 16
The second half opened with a yellow card for Diack for coming off his feet at an early ruck, adding a tenth penalty to Ulster's ever-rising infringement count, and allowing Sexton to add an easy three points. Their numerical disadvantage did not last long, however, with Nacewa - in his last ever appearance for his adoptive province - sinbinned for a high tackle on Jackson as the outhalf launched an inspired solo raid in the Leinster '22'.
Pienaar dispatched the subsequent penalty with minimal fuss, and added three more points on 50 minutes after Ian Madigan had been penalised for holding onto the ball under Tommy Bowe's tackle. With Diack back into the fray five minutes later, Ulster continued to chip away at the Leinster lead, Pienaar kicking over three more points for his fifth penalty of the encounter.
With the game hanging in the balance at the hour mark, Sexton eschewed a further three points with a kick right into the corner, and after three minutes of exemplary recycling, Ireland skipper Jamie Heaslip dived over the whitewash under a trio of tackles to edge his side back up to a nine-point lead. Sexton's conversion attempt bent just wide of the posts, leaving Ulster with a glimmer of hope for the remaining 15 minutes.
An exceptional 50-metre penalty from Pienaar restored the six-point margin on 70 minutes, and strong scrummaging on the Leinster 10-metre line moments later won Ulster a well-deserved penalty as a desperate Jamie Hagan leapt off his feet. Pienaar judiciously kicked to touch, but as Ulster probed from their well-worked lineout, the Leinster defence stood strong and resilient until replacement Iain Henderson eventually infringed.
Leinster won their lineout and kept the ball well as the clock ran down to zero, Brian O'Driscoll belting into touch as soon as possible, to secure his side's first domestic league play-off victory after three consecutive years of falling at the final hurdle - a fitting send-off for head coach Joe Schmidt as he prepares to take the helm of the Ireland national side.
Full-Time Score Ulster 18 Leinster 24
(15 - 9) J Payne; A Trimble, D Cave, S Olding, T Bowe; P Jackson, R Pienaar
(1 - 8) T Court, R Best, J Afoa, J Muller (c), D Tuohy, R Diack, C Henry, N Williams
(16 - 23) R Herring, C Black, D Fitzpatrick, I Henderson, M McComish, P Marshall, M Allen, P Nelson
(15 - 9) I Nacewa; F McFadden, B O'Driscoll, I Madigan, A Conway; J Sexton, I Boss
(1 - 8) C Healy, R Strauss, M Ross, L Cullen, D Toner, K McLaughlin, S Jennings, J Heaslip
Replacements (16 - 23) S Cronin, J McGrath, J Hagan, Q Roux, R Ruddock, J Cooney, A Goodman, D Kearney