But a second half surge from Leinster - who included several first team squad players - saw them claim the cup victory and left Stanley to reflect on the small errors that had proved costly in Ireland.
"We are disappointed and the lads have put their hands up afterwards - individual errors cost us," said Stanley.
"We thought getting a score just before half-time, to give us a 15-10 lead at the break, would be crucial. We had weathered the early storm and got into the pattern and shape we wanted to and were effective.
"We kept hold of the ball reasonable well and got the scores on the back of it. We went in at half-time very positive and looking to come out and make a big start.
"But we conceded soft tries that we are very disappointed about. One from a lineout and one from an inside pick and go; ultimately that was the difference between the teams.
"Games are turned on small margins and when we look back we know what we need to correct.
"Full credit to Leinster; they took their opportunities well when they arose and had some very good players in the back line who, when the game broke out, were very good at one-on-ones to make things count.
"We can take positives from it though, when we did get out patterns right we caused them a lot of trouble. We put men in space and showed what we could do.
"Unfortunately, we didn't get back into the game until the last 15 minutes when we came on strong again. We have got to do our best to fix it now against Plymouth Albion next weekend."
Stanley also praised the impact of younger players such as Nile Dacres, Joel Dudley, James Whybrow and James Clutterbuck who all got valuable game time in Ireland while centre Alex Grove made his playing return.
He said: "I was pleased with the young lads as they did well. We had a few come off the bench who showed up strongly.
"This cup is a great format to get young players like them out on the pitch and playing alongside the more experienced players. They will be better players for getting the opportunity to come to a place like Donnybrook and play against a strong Leinster side.
"It was also good to see Grovesy back," he added. "Getting a quality player like that back in the panel is obviously a big plus for us."