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.
Leinster lined out with twelve of the team that beat Toulouse in that pulsating Heineken Cup semi-final two weeks ago, the other inclusions being McFadden, ex-Ulsterman Isaac Boss and Shane Jennings.
Ulster secured their play-off spot with a well-judged away victory over the Dragons last Friday. Dan Tuohy (hamstring) was ruled out for the trip to Dublin, with Tim Barker taking his place in the second row.
Brian McLaughlin's charges 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 quickly got on the front foot, a lovely improvised pass from Brian 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. 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 gap remained at eight points as Sexton's conversion attempt slammed off the right hand post and Ulster, although gaining momentum, were let down by handling errors.
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 injury-time penalty.
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 having suffered a knock to his right knee.
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, whose scrum was solid throughout, 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 called ashore.
With 18 minutes remaining, Pienaar opened Ulster's scoring account with a simply stunning penalty goal from close to the visitors' 10-metre line.
The next score was hugely important and it went to Leinster in the 72nd minute. Fitzgerald managed to worm his way past Pienaar and Wannenburg before showing great pace to scoot in under the posts for the clinching try. McFadden added the extras.
Credit to Ulster, they played right to the final whistle and players such as Chris Henry, Johann Muller, Spence and replacements Jerry Cronin and Conor Gaston were heavily involved in a late series of attacks.
However, there was no reward for the visitors as Leinster saw out the win to book their place in the May 28 grand final - quite an achievement considering they lost three of their first four league games back in September.
|Leinster Rugby Score Card|
|Ulster Rugby Score Card|