Fergus McFadden's 25th-minute effort was the only try of a tightly-contested first half, supplemented by two Jonathan Sexton penalties to give the hosts an 11-0 half-time lead.
A monster penalty from Magners League Player of the Year Ruan Pienaar got Ulster off the mark, but Luke Fitzgerald's late converted try leaves Leinster just 160 minutes away from a potential Magners League and Heineken Cup double.
But they have a few injury concerns, including Brian O'Driscoll after the centre was taking off after suffering a a leg injury just eight days before they face Northampton at the Millennium Stadium.
Ulster had completed the league phase in impressive fashion, claiming nine wins in their last 11 outings in the competition.
But injuries to Stephen Ferris, Paddy Wallace, Andrew Trimble and BJ Botha robbed them of key personnel and their provincial rivals proved too strong for them for the third time this season.
Leinster lined up with 12 of the team that beat Toulouse in their pulsating European semi-final two weeks ago, the other inclusions being McFadden, ex-Ulsterman Isaac Boss and the fit-again Shane Jennings.
Dan Tuohy (hamstring) was ruled out for Ulster, with Tim Barker taking his place in the second row.
Leinster quickly got on the front foot, a lovely improvised pass from O'Driscoll sending Sean O'Brien storming into the 22 and Sexton clipped over the ensuing penalty.
Ulster leaked four tries in the opening 36 minutes of their most recent visit to the RDS, but their defence was much better this time, although they had a let-off when Fitzgerald knocked on a pass from the rampaging Richardt Strauss.
Sexton was off target with a penalty from just inside the Ulster half before Jamie Heaslip, Cian Healy and Isa Nacewa sparked a long range attack which ended with a crucial covering tackle by Nevin Spence on O'Brien.
The men in blue kept the pressure on, opting for a close range scrum from a penalty and, with Ulster caught short for defenders out wide, O'Driscoll and Nacewa put McFadden over in the right corner.
The lead remained at eight points as Sexton's conversion attempt slammed off the right hand post.
Ulster had been frustrated by a series of handling errors but they executed well late in the half, Pienaar initiating an attack which saw Adam D'Arcy find touch five metres out from the Leinster try-line.
But the hosts held out and a Heaslip and O'Brien-inspired break set up Sexton for a successful penalty right on half-time.
With the Heineken Cup final just a week away, Leinster withdrew Boss, Mike Ross and O'Driscoll after they picked up slight injuries - the Ireland captain limped off after taking a knock to his right leg.
The strong-running Robbie Diack helped tee up Ian Humphreys for a long range 45th-minute penalty which he drew just wide of the uprights.
Simon Danielli, Pedrie Wannenburg and Spence carried well as Ulster started to find some rhythm in attack, with Pienaar probing from much quicker ball.
Darren Cave was involved twice before Leinster won a penalty at the breakdown and as the physicality went up some notches, Sexton and a hobbling Strauss were also withdrawn from the fray.
With 18 minutes remaining, Pienaar opened Ulster's account with a stunning penalty goal from close to the visitors' 10-metre line.
The next score was hugely important and it went to Leinster, with Fitzgerald worming his way past Pienaar and Wannenburg before showing great pace to scoot in under the posts.
McFadden converted and Ulster's late series of attacks went unrewarded as Leinster booked their spot in the May 28 decider for a second successive Magners League final appearance.
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