Connacht opened their season with a 29-23 win at the Sportsground over Newport Gwent Dragons while Leinster were beaten by a Damien Hoyland brace of tries in Edinburgh.
Leinster bounced back with wins over Cardiff Blues and the Dragons before suffering a second defeat of the season at Scarlets.
Connacht meanwhile were downed by a single point at reigning champions Glasgow Warriors before going on a six-game winning streak through October and November and serving notice of the force they were to become.
Fergus McFadden kicked 13 points as Leinster overcame the Warriors in round five and they were unbeaten in five when they met Connacht for the first time of the season on January 1.
Josh van der Flier's second-half try proved decisive on that day as an in-form Leinster leapfrogged Connacht to move second in the Guinness PRO12 table with a 13-0 victory at the RDS Arena.
Connacht were beaten again by Scarlets the following week but bounced back with a run of five straight wins ahead of their re-match against Leinster.
The Irish province travelled to Galway on March 26 after losing 12-6 to Glasgow in a re-arranged round nine fixture.
And there was to be no double success over their Irish rivals for Leinster as league leaders Connacht's sealed a narrow 7-6 victory thanks to Kieran Marmion's try.
With four games to go, both sides looked set for a top four spot and it was Leinster who finished as league leaders thanks to victories over Edinburgh, Benetton Treviso and Ulster.
Connacht also managed sealed a home semi-final, despite losing to Benetton Treviso, with a 35-14 victory over Munster and a second victory of the season over Glasgow.
In the semi-finals, tries from Jamie Heaslip, Isa Nacewa and Sean Cronin saw off an in-form Ulster as Leinster clinched a 30-18 win.
The hosts were clinical in attack and immovable in defence as they maintained their unbeaten record at home in the Guinness PRO12 this season at the RDS.
Meanwhile, the boot of AJ MacGinty booked Connacht a place at BT Murrayfield as the men from Galway edged past reigning champions Glasgow 16-11 at the Sportsground.
Those results meant that the wait goes on for an away semi-finalist to reach the Guinness PRO12 Final, although with Connacht in the mix for the first time, this year's showpiece could be the most intriguing yet.
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