In recent seasons, Ireland have held the edge over their Celtic rivals thanks to seven wins in eight encounters since October 2001.
The one that got away was Wales' red letter day at the Millennium Stadium in 2005 when they beat O'Sullivan's men 32-20 to claim the Grand Slam.
Roll on three years and O'Sullivan feels that although Gatland, his predecessor in the Ireland job, may have got Wales ticking again, they are sure to face a baptism of fire when they visit Croke Park for the first time.
"You've got Wales coming to town in a couple to weeks. They're on a roll at the moment with three wins out of three but we're also building some momentum ourselves," O'Sullivan admitted.
"In fairness to Warren, he's got Wales on the right foot, they're going forward.
"We saw a Welsh team back in 2005 when they got that sort of start to the championship, their confidence was up and they really were a handful.
"We lost the Triple Crown to them in Cardiff and they went on to win the Grand Slam that day and deservedly so.
"We see that Welsh team back now on the pitch with the same sort of pep in their step and the same confidence and to be fair to Warren, he's induced that very, very quickly in them.
"We know they're always extremely difficult to beat in Cardiff. But they've got to come to Dublin, this is our backyard and we're not going to give anything up very easily in a couple of weeks' time. It's all to play for for us."
Brian O'Driscoll and company clawed back some more self belief by hitting Scotland for five tries when the sides met on Saturday.
There were some glimpses of Irish attacking magic in that game and O'Sullivan reckons the Croke Park crowd could be treated to more of the same in two weeks' time, especially if the rain stays away.
"It really could be the match of the tournament," said the Corkman after his 50th win as an international coach.
"Because of the direction we're going in and because of our own growing confidence, we could have a cracking game on our hands.
"Both ourselves and Wales like to run and if it's a dry day at Croke Park, it will be all bets are off and a great spectacle."