The Durban-based side completed their three-game overseas tour with a loss after also going down 35-19 to the Glasgow Warriors and recording a 17-17 draw against perennial strugglers the Dragons from Wales.
Defending champions Leinster, bolstered by the return of several of their British & Irish Lions players, was always going to be a tough task, but it will be the manner of the defeat that will be most worrying for Plumtree.
“Obviously really disappointing,” he said. “We weren’t good enough and we’ve got a massive job ahead of us the next two weeks, a couple of home games, and we’ve got to work really hard and get into this competition.
“It was always going to be a pretty tough tour for us, with a lot of disruptions, but there’s no excuses now. We’ve got to get back to South Africa and play really well in front of our fans.
“The team’s disappointed, and collectively we’ve got to take responsibility to be better over the next two weeks and that’s what we’ll do.”
The Sharks host Ulster in Durban on Saturday and then face the Scarlets a week later before a break in the URC season for the November international window.
Plumtree had little positive to say about his side run-out in Dublin, where wing Ethan Hooker scored their only try.
“It was a tough night for us. We knew the start was going to be massively important. They came out fast and we just didn’t get the contestable kicks in terms of regaining the ball in the air.
“We had a few soft moments defensively that allowed them to build that scoreboard pressure early.
“The last 10-15 minutes of the first half, we got back into the game, but it was really a case of catch-up rugby. In the second half, we did our best, but their defence was just too good.
“Once we gave them that lead, it just gave them that confidence to defend really well, and we just didn’t fire enough shots.
“That’s the message to everyone: we’ll keep fighting hard and make sure we get it right over the next fortnight.”