"I don't think there was anyone in the dressing room who was surprised that we had beaten England," the 29-year-old fly-half said.
"That's four out of six for a lot of us since we came in 2000.
"We'd have been very disappointed if we hadn't beaten them but at the same time it was away from home and with their backs against the wall, so obviously the lads are happy."
The Munster stand-off - who contributed 13 points to his team's cause - is adamant Ireland now have a group of players their nation can be proud of.
"You look at the quality we have all over the pitch - I couldn't be happier with the players I've got around me.
"There's huge satisfaction because some times people don't recognise there are some quality players in green, they're just happier to pick out their weaknesses.
"But we've shown that our strengths outweigh our weaknesses."
O'Gara singled out the performance of match-winning winger Horgan, whose late try sealed a dramatic 28-24 success for Eddie O'Sullivan's men.
"Shaggy's (Horgan) an incredible finisher. I don't think anybody on the pitch apart from him would have scored that last try," O'Gara said.
"It was an exceptional score and he probably still had a foot to spare so huge credit must go to him.
"It was one of the most important tries he'll ever have scored."