Leinster secured a home semi-final in the Pro12 play-offs with a 37-24 victory at Irish rivals Connacht, while Munster also confirmed a last-four place with a dramatic late win at Ulster.
Last season, Leinster were beaten by Connacht in the Pro12 final, but Leo Cullen's men remain well in contention to go one better this time around, and defeat for Ospreys on Saturday coupled with Scarlets failing to secure a bonus point against Newport Gwent Dragons means they cannot finish lower than second.
Cullen made 11 changes to his team for the encounter and they led by just one point at the break as, despite tries from Ross Byrne, Adam Byrne and Rory O'Loughlin, Craig Ronaldson's boot kept the hosts in contention.
A Dave Heffernan try briefly had the soon-to-be deposed champions ahead, but scores from Rhys Ruddock and Barry Daly put Leinster into a lead they did not relinquish.
Munster are also confirmed for the play-offs, but only after Dave O'Callaghan's late try earned a 22-20 victory over Ulster at Thomond Park.
Ulster's Rory Best and Munster's Angus Lloyd, on loan from Ulster, exchanged early tries as the match headed for the break at 10-10, but it was in the second half where the drama unfolded with the lead changing hands four times.
Keith Earls' try put Munster in front only for Luke Marshall to put the visitors back ahead. But there was one final twist as O'Callaghan crossed with the clock winding down and Tyler Bleyendaal struck the winning conversion to put Munster into the last four and dent fifth-placed Ulster's own play-off aspirations.
It was 'Judgement Day' in Wales, with two matches taking place at the Principality Stadium.
Third-placed Scarlets defeated Newport 21-16 thanks to second-half tries from Jonathan Davies and Liam Williams to cement their place in the top four with two rounds of the regular season remaining.
Ospreys' hopes took a hit, though, as they were turned over 35-17 by Cardiff Blues. They remain in fourth and one point ahead of Ulster.
The other match on Saturday saw Benetton Treviso move off bottom spot with a 21-6 defeat of Edinburgh.