Ulster coach Dan McFarland says his team’s mistakes opened the door for 14-man Leinster to fight back from 22-3 down to win 38-29 at the RDS Arena.
It is Ulster’s second defeat of the season by Leinster, who maintained their 100% record in the URC.
“We made a few mistakes in the middle of the field which allowed Leinster to get on the front foot,” said McFarland.
“We have to look at the mistakes we made in the third quarter and make sure we iron them out.”
Leinster prop Cian Healy was sent-off on 21 minutes and Ulster took charge with three tries but Ronan Kelleher went over for the hosts just the break and they went on dominate the second half.
The leaders capitalised on Ulster being temporarily reduced to 13 men after yellow cards for James Hume and Nick Timoney.
Ulster missed a golden chance of beating Leinster in Dublin for the second straight season and they will now turn their focus to Europe with next weekend’s Champions Cup opener against Sale.
“There’s a lot of momentum with Leinster going in at half-time with the try and the mistakes allowed them field position and to really get back into the game,” added McFarland.
“Obviously the two yellow cards were costly. From my perspective the lads gave a lot, they worked hard but we obviously need to be better in that period.
“To get three tries ahead when they went down to 14 we put some nice play in, forced a transition try and a couple of good tries in their 22 with our power play – we’ll take a lot from that.
“Nobody comes here to make mistakes and when you play a side as good as Leinster you are put under a bit of pressure or quite a lot of pressure and it’s very difficult to play.
“There’s no room for licking our wounds – we’ll get back to it on Monday and have a little bit of a review, then have a good preview on Sale and get our heads down and work hard at that.”