To make matters worse, Will Genia appears to be in major doubt for the third game in Sydney after leaving the field in the first half with what looked to be possible arm fracture.
The Wallabies appeared to be cruising after Kurtley Beale scored a try after just 90 seconds but Ireland made the most of a sin-binning for Marika Koroibete soon after, scoring 13 points with the winger off the field.
The Wallabies fought back by scoring through a penalty try but with Sexton running the show, the Irish were able to keep up the pressure and build their narrow halftime lead with a try to powerhouse prop Tadhg Furlong after the break.
Sexton's fourth penalty goal in the 66th minute appeared to ice the result but the Wallabies gave the crowd of 29,018 a grandstand finish by scoring in the 78th minute through Taniela Tupou.
Almost fittingly, though, an Australian mistake at the ruck on the siren ended proceedings as the Wallabies were still attacking for a try to draw or a win.