Leo Cullen says he and his Leinster squad will be having "a hard look at ourselves" after a frustrating day at the office in Ravenhill.

Ulster ran out 17-13 winners on a gritty evening in Belfast, a result that dropped Leinster to second in the Pro12 table and set up a semi-final against in-form Scarlets in Dublin in two weeks' time.

Disappointed head coach Cullen acknowledged his side appeared somewhat distracted by the challenges that lay ahead as they came out second best in the physical battle.

"The performance is probably the most disappointing part," Cullen told RTÉ Sport. 

"We were just a little bit off today. I thought Ulster were quicker on to most things, particularly in the first half, which gets us into the position where we are chasing the game.

"So, a little bit of a frustrating watch. The players are frustrated. You come to the end of the season and there's lots of distractions and we were definitely a little bit off today, which is really disappointing from our point of view.

"We had a lot of possession and put a lot of pressure on their lineout and set piece in general, but we just weren't able to dominate enough in our carries and Ulster were good at holding us up and slowing us down.

"We'll have a good look at it and have a hard look at ourselves."

Cullen did pay tribute to Ulster, who delivered the perfect send-off to long-serving players Ruan Pienaar and Roger Wilson, even if the win wasn't enough to force their way into the top four.

"You have to give Ulster a little bit of credit," Cullen said. "They looked emotionally charged with a lot of guys moving on and they just showed a lot more intent than we did."

With attention now turned towards Scarlets and the chance of ending the season with some silverware, but Cullen knows the Welsh outfit will be a tough nut to crack.

"We've had two really good games against Scarlets," he said. 

"We lost over there in a high-scoring game and beat them back at the RDS. They're a quality team. They lost the first three games of the season but gone from strength to strength since. It's going to be a really tough game."