Dempsey, 30, will be winning his 66th Test cap against Australia, while it will be a 63rd for the 29-year-old O'Gara.
Ireland coach Eddie O'Sullivan defended Ronan O'Gara this week after the Munster player was on the receiving end of some criticism for his missed tackle which led to Luke McAlister's clinching try in the second Test against New Zealand.
O'Sullivan said: "Ronan's place-kicking on this tour, and of late, has been phenomenal. He has nailed every kick, and a lot of them were real pressure kicks. His distribution has been good, his line-kicking has been good, but we know that he is susceptible to mismatches in physicality, and he knows that as well."
Referring to the McAlister incident, he added: "Ronan had brought off some very good stops in the match. I think he tried to go low, slipped off it and McAlister got in.
"I don't want to say that if he put his left foot there, or his right foot somewhere else that there could have been a different outcome. The important thing is that they scored, and Ronan put his hands up and admitted his mistake. We paid a big price for it, but I won't hang him out to dry on the basis of one tackle. It was a big moment, but it would be a harsh thing to do to a player."
Ronan O'Gara lines up a shot at the posts at the 43,500-capacity Subiaco Oval. The Corkman's haul of two conversions and a penalty in last weekend's 27-17 defeat to the All Blacks brought his tally as Ireland's record points scorer to 627 points.
Girvan Dempsey started at full-back the last time Ireland met Australia at Perth's Subiaco Oval in June 2003. The Irish back line for that 45-16 defeat read as follows: Dempsey, James Topping, Geordan Murphy, Kevin Maggs, John Kelly, David Humphreys and Peter Stringer.
Ronan O'Gara's opposite number Stephen Larkham has talked up the Irish team ahead of this weekend's clash - Ireland's fourth meeting with the Wallabies in little over three years.
The Australian out-half said: "It's definitely the best Irish side I've played against over the past 10 years. Not just because of the form they've shown over the past couple of weeks but because of the quality of player they can put on the field.
"Their back line - I don't know what the stats are - but they have a ridiculous number of players with over 60 Tests. So there is a lot of experience out there on the paddock. They know how to play good football."
Larkham also hailed Ronan O'Gara as the best kicking number 10 in the world at present. He added: "I think O'Gara's kicking game is the best of any number 10 in the world at the moment. England really struggled with their kicking (against us). It wasn't on song.
"But I imagine with O'Gara, our back three (full-back Chris Latham and wingers Lote Tuqiri and Mark Gerrard) will have a much bigger job ahead of them on Saturday."
**All photos by Billy Stickland of Inpho Photography**