To hold onto their title, and challenge for an unprecedented three wins in a row in world rugby's top club tournament, Joe Schmidt's men need to pick up a maximum 10 points from their two remaining Pool 5 matches, home against the Scarlets and away in Round 6 against Exeter Chiefs.
Former Lions captain O'Driscoll had recently suggested that he wouldn't be ready until the last week in January, while Kearney had been thought to be a week behind in terms of medical schedule.
But Schmidt has had both on the training field in Dublin this week and preparing for action in the PRO12 clash with Edinburgh at Murrayfield.
"The problem is that there is not a lot of time to knit ourselves back together when we have to chase big games, Scarlets and, then, Exeter away.
"It would have been nice to get a run in of two to three games with those guys available and challenging for spots, but it's not a perfect world unfortunately."
Full back Kearney - who will make his 122nd appearance for the province - is named in a pacy-looking back three alongside Fitzgerald and Fionn Carr, with O'Driscoll linking up in midfield with Gordon D'Arcy.