Munster handed Leinster their second defeat of the season with a 26-17 victory, while there were also Pro14 wins for Connacht, Benetton Treviso, Edinburgh, the Cardiff Blues and Dragons.
The defending champions were not helped by losing James Lowe to a red card for a dangerous tackle in the closing stages of the first half. This after both tighthead prop Tadgh Furlong and loosehead Cian Healey were yellow-carded.
However, it was left to the home side to punish Leinster for their ill-discipline, which they did through tries from flank Chris Cloete and wing Keith Earls, while flyhalf Joey Carbury kicked 14 points to help them keep their unbeaten record at home.
Munster opened the scoring when Cloete went over from the back of a driving maul, with Leinster responding through a penalty from Johnny Sexton. The visitors were handed a massive blow with Furlong already in the sin-bin for a high shot on Cloete, when Lowe was shown a straight red after colliding with Andrew Conway.
The home side capitalised, Carbury kicking three penalties to see them into a 16-3 lead just after half-time, but Leinster then hit back when hooker James Tracy burrowed over from close range. An intercept try from Earls seemed to put the game to bed, but Leinster grabbed a late consolation through replacement flank Max Deegan.
In other results, flyhalf Jaco van der Walt scored 11 points with the boot as Edinburgh did the double over the Glasgow Warriors, beating their rivals 16-8. Edinburgh prop Pierre Schoeman was named Man of the Match.
Connacht bounced back from their heartbreaking loss to Leinster last weekend by handing Ulster a 21-12 defeat for their sixth win in seven games. Treviso won their second game against Italian rivals Zebre 28-10, while a brace from centre Ray Lee-Lo saw the Cardiff Blues end the Scarlets’ 27-month unbeaten home record with a 34-5 victory.
In the only match on Sunday, the Dragons survived a late comeback to beat the Ospreys 23-22, with Ospreys flyhalf Sam Davies dramatically missing a 79th-minute penalty which would have given the visitors a last-gasp victory.
Round 12 results:
Connacht 21 Ulster 12
Treviso 28 Zebre 10
Glasgow Warriors 8 Edinburgh 16
Munster 26 Leinster 17
Scarlets 5 Cardiff Blues 34
Dragons 23 Ospreys 22
Photo: Diarmuid Greene/Sportsfile via Getty Images