Ulster skipper Rory Best admitted Leinster had taken their chances when it mattered in the 24-11 reverse in Dublin on Saturday evening.
The poor run of form on the road continued and Leinster jumped from sixth to fourth place with the victory as Ulster slipped to fifth spot - a point behind their arch rivals.
Ulster totally dominated the first half but went in at the break trailing 9-6 in spite of that.
Pienaar kicked two of three penalty attempts - the crossbar denied his first long range effort.
But having got their noses in front on two occasions, ill discipline crept in to the Ulster ranks once again and the lead was cancelled out on both occasions.
Lock Dan Tuohy’s needless penalty give away under the posts in the final minute of the first half was to prove costly - Madigan converting his third penalty of the half to make it 9-6 and then the outhalf scored the try which turned the game three minutes into the second half.
Best, who thought it was a good game of rugby, said:
“I think they (Leinster)were definitely more clincial.
“We were going well and then the yellow card obviously cost us a bit.”
Madigan’s try came when it appeared he was kicking a penalty for touch, but took a quick tap and managed to wriggle past Callum Black and Roger Wilson for a try.
It was a costly lapse for Ulster.
Best said: “As much as anything it was that lapse in concentration - that penalty five metres out - that was a soft try by us and a fairly pivotal moment in the game.”
Ulster found themselves on the front foot so often in the game, but then ill discipline was to halt them maing furthing inroads at key times.
Best said: “We just could not get the momentum we needed.
“Once we got going with ball in hand we would give away a penalty in attack and then give a few away in defence as well.
“Both teams discipline was similar, they may have been a bit more streetwise with theirs. We got on the wrong side of the ref and allowed them to get field position.
“In the first half we controlled the game really well and the real difference between the sides today was how to take chances when you have that momemtum - they did.”
Ulster’s dip in form this season, particuarly on the back of a number of successful seasons, has been a concern, but Best believes they are still a top four PRO12 side.
“It is difficult wehn you have so many injuries. We probably do not have that strength in depth other teams have.
“I do not think we are a team in transition. This is a good step for us and we ran out of steam. There are a lot of elements of that game we will be happy with. There are also elements we will not be happy with at all.
In terms of transisition we are definitely a top four side, we just need a bit more consistency througout that 80 minutes.
There were definitely things we could take a lot of heart from today.”