Young centre Luke Marshall, who made a couple of strong runs in his fourth appearance since February, was annoyed at himself for allowing Cooper to slip through for that 47th minute score.
"I was looking back on it on the big screen and he probably was my man. It was a lapse in concentration and playing new combinations, not the defence I'm use to, but no, I put my hand up, my mistake, and it was a pretty costly error, early on in the second half," explained the 22-year-old.
Despite the 32-15 defeat to the Australians in head coach Joe Schmidt's second game in charge, Marshall says there is no need to push the panic button. He hopes the squad learns their lessons quickly before they take to the pitch against New Zealand next Sunday.
"It's early on in Joe's reign. We are learning new things with him, a couple of new combinations as well, but (we're) still very disappointed with most of our performance.
"There were positives to take out of the game as well and we will look at the positives and learn from the mistakes.
"We need to recover quickly after this. We don't have any time to feel sorry for ourselves. We just need to forget about the result, study the game, study the mistakes and learn from them. Put together a game-plan to hopefully beat the All Blacks next week."
Four penalties from Sexton brought Ireland to within three points of Australia at half-time (15-12), but Ewen McKenzie's men made Ireland pay for a series of defensive lapses.
Marshall added: "At half-time I thought we could definitely win the game. I thought there was a swing in momentum at the end of the first half, so we were pretty confident going into the second half. But we gave a try away early on and that killed us a bit.
"A few things in our defence. We got caught a couple of times on the short side and obviously with runners like (Israel) Folau and Quade Cooper you can't do that.
"We struggled to get the quick quality ball off the first couple of phases near the goal-line, (but) we didn't run in the wide channels as much as we wanted to."
Marshall, who is aiming to retain the number 12 jersey ahead of Leinster centre Gordon D'Arcy, feels Ireland can turn it around as thoughts turn to the GUINNESS Series finale against the world's number one team.
Looking ahead to that match, the Ulster youngster said: "I don't think we plan to play like that next week. If we do, I don't think there is a point going out on the pitch next week.
"We aim to improve, we are going to aim to win the game. If you are going out not expecting to win, you shouldn't be going out (to play).
"Gordon has been very good in training, especially this week when I got the nod - he was still giving me all the advice, helping me improve and obviously for me as a young player that is key.
"He has so much experience and advice to give and I'm just trying to learn as much as I can. If he gets the nod next week, that is just the way it goes."