Leinster staved off an Ulster comeback to seal a 30-18 victory along with a place in the PRO12 play-off final.
A Craig Gilroy try and two penalties late in the first half brought the visitors back in to the tie, but Leinster maintained their lead for the entire game after Isa Nacewa's try inside five minutes.
Jamie Heaslip and Sean Cronin added to Leinster's tally, ensuring their side's place in the play-off final on May 28.
Ulster did look to find their way back in to the game late in the second half, but another try from Gilroy was too little, too late for Les Kiss' troops.
Full-back Nacewa scored an excellent individual try to open the scoring after being found on the right by Eoin Reddan, and the conversion from Sexton made it 7-0 to the home side in the opening five minutes.
Sexton added another three points from the tee with eleven minutes gone, before slotting again just moments later.
Paddy Jackson finally opened Ulster's account by adding a penalty either side of the half hour mark, bringing his side back to within a score of Leinster.
Ulster then got themselves back in to the tie through Gilroy, who powered his way into the corner under the challenge of Dave Kearney who was unable to take his international teammate in to touch.
Jackson missed the conversion from a tight angle, but Gilroy's score had reduced the deficit to just two points as the teams went in at the break.
A moment of individual excellent from Garry Ringrose saw him dart past a number of Ulster defenders, before Heaslip powered his way over in the corner having been found by a superb Ben Te'o offload.
Sexton took the opportunity to display his expertise from the tee, as he converted from the left hand touchline, before extending the lead with another penalty on 56 minutes.
Leinster secured the victory shortly after the hour mark as replacement hooker Cronin burrowed over from close range, before Sexton's kick took the score to 30-11.
Gilroy kept Ulster's dwindling fire alive as he scored under the posts with ten minutes remaining, allowing Jackson to add the extras with a quickly taken drop goal.
Tries: Nacewa, Heaslip, Cronin
Cons: Sexton 3
Pens: Sexton 3
Tries: Gilroy 2
Pens: Jackson 2
Leinster: 15 Isa Nacewa (c), 14 Dave Kearney, 13 Garry Ringrose, 12 Ben Te'o, 11 Luke Fitzgerald, 10 Johnny Sexton, 9 Eoin Reddan, 8 Jamie Heaslip, 7 Jordi Murphy, 6 Rhys Ruddock, 5 Mick Kearney, 4 Devin Toner, 3 Mike Ross, 2 Richard Strauss, 1 Jack McGrath
Replacements: 16 Sean Cronin, 17 Peter Dooley, 18 Tadhg Furlong, 19 Ross Molony, 20 Jack Conan, 21 Luke McGrath, 22 Ian Madigan, 23 Zane Kirchner
Ulster: 15 Jared Payne, 14 Andrew Trimble, 13 Luke Marshall, 12 Stuart McCloskey, 11 Craig Gilroy, 10 Paddy Jackson, 9 Ruan Pienaar, 8 Sean Reidy, 7 Chris Henry, 6 Iain Henderson, 5 Franco van der Merwe, 4 Pete Browne, 3 Ricky Lutton, 2 Rory Best (c), 1 Callum Black
Replacements: 16 Rob Herring, 17 Kyle McCall, 18 Andrew Warwick, 19 Robbie Diack, 20 Roger Wilson, 21 Paul Marshall, 22 Stuart Olding, 23 Darren Cave
Referee: Ian Davies (WRU)
Assistant Referees: Ben Whitehouse, Sean Brickell (both WRU)
TMO: Jon Mason (WRU)