Sportal.co.nz 26.Mar.2013Getty Images
The men from Auckland ran in three tries to one to lead by 14 points at the break but repeatedly coughed up possession and infringed at the ruck in the second stanza.
Tries to NRL convert Israel Folau and man-of-the-match Bernard Foley saw the Waratahs draw level before Foley nailed a 35 metre penalty goal after the siren to snatch victory.
"At half-time we were reasonably happy, we felt that we needed to tighten up the ruck area, show a little bit more discipline and hang onto the ball - they were the messages at half-time," Kirwan said.
"And then we went out and pretty much did the opposite.
"We had eight errors in the first half and 10 in the second half and then I wouldn't like to see the penalty count against us at ruck time.
"So making 18 errors in a football game and having the penalty count against you is not two things you can do and win at this level.
"We really beat ourselves a wee bit. The Waratahs were really under the pump and played well, they've had a pressured week so they came out and they took their opportunity and fed off what they did.
"So we've got ourselves to blame and got to get back up for next week."
Lock Culum Retallick conceded the final penalty for being offside at the ruck.
Asked his opinion on the ruling, Kirwan replied: "I think there are situations where if the half-back has two hands on the ball it's out but then you really can't take those risks when you're under the pump and it's 27-all, you've just got to wait for the ref to call out.
"If you look at that last penalty you could probably say: 'was it a penalty or not?' But at the end of the day you can't take those risks when you're under the pump like we were, so it's totally our fault."
Kirwan also refused to blame the 30 degree temperatures for his side's second-half collapse and demanded a much-improved effort against defending champs the Chiefs in Mount Maunganui next Saturday.
"It's been like this all summer so these guys have been charging around in this and there's no excuses coming from us," he said.
"Eighteen errors, I think individually we're all going to have to have a look at ourselves because if we're going to do that again next week (against the Chiefs) then you'll be kissing any chance of the top six good bye.
"So we've just got to get out and play our game because when we put it together we looked pretty good."