Saturday's 32-22 win by the Irish in Wellington clinched a historic first series win in New Zealand and moved them top of the world rankings.
"If we played New Zealand again next week, we'd probably beat them again," said ex-Ireland coach O'Sullivan.
"But I think, come the World Cup, things will even up again."
"There is a lot of water to flow under the bridge between now and next year," he added.
Ireland's latest victory over the All Blacks was their fifth win in eight contests against the three-time world champions.