Munster have forced their way into the top four, at the expense of Welsh region Scarlets, while Leinster, Ulster and Glasgow Warriors returning to the playoff fold.
At this stage last year defeat to Ulster at the end of March had seen this year's table-toppers Leinster slip back to second, but they retained that all-important home semi-final.
Glasgow Warriors, who had lost the same fixture 19-15 one year before, were the visitors to the RDS and they suffered a similar fate.
Niko Matawalu's try after a quarter of an hour put the Scots ahead but Jamie Heaslip dotted down soon after, before the boot of Jonny Sexton eased the hosts into the lead.
The fly-half sent four penalties over to Stuart Hogg's one, but the Warriors full-back had the opportunity to provide some late drama.
When Mark Bennett went over five minutes from time, Hogg's conversion would have made it 17-17 but it was not to be and Leinster reached the final.
In the other semi-final Ulster received a late scare from Welsh upstarts Scarlets after dominating proceedings early on.
Tries for Tommy Bowe, Robbie Diack and Tom Court had the hosts 25-3 up five minutes into the second half and coasting to the final.
Gareth Davies and Sione Timani each dotted down inside the final 20 - both converted by Owen Williams, but ultimately Scarlets fell short, losing 28-17.
In an all-Irish final, Leinster got off to a flying start as flanker Shane Jennings crossed the whitewash after just three minutes - Sexton duly converted.
Sexton then proceeded to exchange penalties with Ulster scrum-half Ruan Pienaar, with the South African sending six from six between the posts.
However, the tie was settled shortly after the hour mark when Heaslip doubled his playoff try tally to hand Leinster their third RaboDirect PRO12 title.
The RaboDirect PRO12 playoffs are building to an exciting conclusion and you can get involved - buy your tickets to Leinster v Ulster at http://www.ticketmaster.ie/event/18004CA7F35631BF?did=retsneil&brand=ie_leinster or http://ulsterrugby.com/News/LatestNews/TabId/149/ArtMID/793/ArticleID/1501/Play-Off-Ticket-Information.aspx
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