Ian Madigan scored 19 points, including a sublime opportunistic try, to lead his side to a 24-11 triumph over their inter-provincial rivals at the RDS Arena as Leinster moved above Ulster in the GUINNESS PRO12 table.
Although the fly-half's tap-and-go effort came with Ulster lock Dan Tuohy in the sin bin, Cullen admits his side should have pressed home their advantage further before Jack Conan's late score.
And while Madigan may be getting deserved plaudits for his moment of magic, Cullen was equally impressed by the all-round performance of young back row Conan.
"We were happy with the way the lads prepared and we're happy with the way we're moving," said Cullen. "We lost Shane Jennings and Jamie Heaslip to injury but the replacements did well.
"We put Ulster under pressure in the first half and we felt they were killing the ball, and we had some cheap turnovers.
"But we started the second half well with their man in the bin and Ian Madigan took his try well.
"We needed to push on when Dan Tuohy was in the bin, but the lads battled back well. We're getting closer and closer and Jack Conan really deserved his try.
"He's worked really hard and he deserves his place in the team. He's really diligent and he has had a taste of it in the first team now and it's about pushing on to another level."
Click here to view the best images from Leinster's derby triumph over Ulster
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