But the Dragons rallied with three tries in a spirited second-half fightback which resulted in a losing bonus point.
Turner said: "We can't get things so defensively wrong against a side of London Irish's quality and expect to win games.We gave the players an old-fashioned team-talk at half-time and, to be fair to the players, they responded in the second half and played some good stuff.
"Our attitude had to be right because of the amount of injuries we had and we had a lot of young players from the Academy out there. It was a tough learning curve for them but it will help us in the years to come. I was pleased with our second half performance but we have to grow up a lot quicker.
"The way we played in the first half was unacceptable: we conceded soft tries. We started off the match well but we have to get out of the situation where if we don't score, they will.
"We gave them too much space and momentum and quality players like Shane Geraghty and Peter Richards will punish you.
"I've always said that if you reach the quarter-finals of Europe, it's the last man standing because you need a strong squad. But I'm pleased with our progress although we got what we deserved because of the way we defended.
"It's important we win next week at Rodney Parade. It doesn't matter whether it's Biarritz or Bucharest, our crowd demand we win. So we will work hard and make sure we're at our best against Perpignan."