Ulster earned a home semi-final against the Scarlets by seeing off Cardiff Blues 37-13 at a wet Ravenhill to finish top of the RaboDirect Pro12 table.
Two second-half tries from Andrew Trimble and a late score for 100 cap man Robbie Diack helped Ulster on their way after Mark Anscombe's side had trailed 13-10 at half-time against a Blues effort which petered out in the second half.
It started badly for the hosts when Dafydd Hewitt scored by the posts after three minute, but that was their only try of the game, as Rhys Patchell scored a goal, a penalty and a drop-kick in the opening period before Ulster claimed the lead and never looked back.
Leinster rounded off the league phase of the with a bonus point 37-19 win as the Ospreys relinquished their grip on the trophy. The Welsh region stunned the home support when they won a dramatic final at the RDS last May, but they were unable to repeat the feat on a rain-soaked night in Dublin.
A brace of Andrew Conway tries catapulted Joe Schmidt's men into a 14-0 lead, but Dan Biggar and Ben John replied as the Ospreys cut the gap to two points. Jonathan Sexton landed an injury-time penalty to give the hosts a 17-12 interval lead, and Conway's hat-trick score further strengthened their hand.
Sexton popped over two more penalties, either side of a Tom Isaacs try, before Cian Healy came off the bench to run in the province's fourth touchdown late on.
Having lost the last three league deciders, Leinster have unfinished business with the Pro12 play-offs and they will entertain third-placed Glasgow Warriors at the RDS next Saturday.
Munster overcame an eight-point half-time deficit to win 27-25 and ensure minnows Zebre finished their campaign without a single victory. The Italian side looked on course to end a disastrous season with a flourish when they went in at the interval 22-14 ahead.
However, Munster, who themselves have an endured a forgettable few months, hit back in the second half and BJ Botha's 65th-minute try, which was converted by Ian Keatley, gave the visitors the points.