O'Gara could also have snatched a late draw but his conversion attempt hit the post as Ireland fell to their fourth consecutive defeat.
But despite the loss O'Gara has expressed his pride at becoming Ireland's third Test centurion.
"I'm very proud to have won 100 caps," said O'Gara.
"It's great to have followed Brian O'Driscoll and John Hayes into the history books. They're two good friends and great players.
"I'm delighted to achieve it. It was a big day for my family and for Munster too. It's a good achievement to reach 100 caps because very few players have done it. I'm very lucky.
"I enjoyed having an impact but it's just a pity it didn't change the outcome because I play sport to win."
Only 35,515 turned out to see Ireland in their first Test at the Aviva Stadium and O'Gara has called on the Irish fans to dig deep and support their team.
"It's strange, the whole economic situation has affected people really badly, more than people appreciate," continued O'Gara. "It's probably only going to get worse."
"I don't know how many were there but there were empty seats all over the stadium. It's probably a sign of things to come.
"Next week I don't know how many thousand will be there for Samoa. It's reality and is something everyone has to look at in terms of getting pricing right.
"We need the supporters - it's because of them that Ireland have been so good for so long. At the old Lansdowne Road there was an unbelievable atmosphere, so passionate and driven.
"We have to get those days back. Emotionally Ireland's greatest tool has been their fans. They've been so good to this team and we need them back. We need them more than ever. We miss them."