If you're not starting at 10 for your club, you can't play international rugby - Ronan O'Gara
Munster's outhalf will make his first Six Nations start inside two centres making their debut in the tournament but O'Gara has backed Joe Schmidt in his selection.
Ian Madigan had been widely expected to deputise for the injured Johnny Sexton, but his lack of rugby at 10 this season and the familiarity that Conor Murray and Keatley enjoy at Munster, mean he starts on the bench.
"It's a massive opportunity, a day he has waited for all his career. He's gone a roundabout way to get it," he told Game On on 2FM.
"It's blatantly obvious to me that unless you're starting week in and week out for your club team, you cannot play international rugby, especially in a number 10 jersey. It's so crucial in terms of managing games and controlling games and it goes up another few notches from Pro12 to Test match.
"The combination of Conor Murray and Ian Keatley is (Schmidt) looking for stability, they'll look to turn the Italians in the first 30 minutes.
"From a coaching point of view, the impact that Madigan brings off the bench and with Tommy O'Donnell is probably interesting."
Ireland will need a victory to give themselves a chance of retaining their crown but O'Gara feels that the Italians may have more at stake in the Stadio Olimpico on Saturday.
"Italian rugby is very much in danger of slipping away. It has badly deteriorated, especially in the last 12 months," he added.
"The club game is non-existent and now it's a massive Six Nations for them.
"They have won one out of their last 10 games and they are going to have to come out of the traps and start very well."