Horgan's try was his 25th in European competitions and he became only the second Leinster player to achieve that milestone after Brian O'Driscoll who continues his centre partnership with D'Arcy.
In the pack, Kevin McLaughlin makes his 11th appearance of the season and starts in the back row alongside O'Brien and Jamie Heaslip, who is one of 13 full Ireland internationals in the 23-man squad.
Speaking ahead of Saturday's Celtic clash, Leinster consultant coach Alan Gaffney said: "The boys want to perform. We were very disappointed after the London Irish game, not only with the result, but with our performance.
"We'll go out on Saturday with a positive frame of mind, wanting to play good rugby.
"Shaunie (Berne) hasn't played a lot at number 10 for a few years. He wasn't the first picked 10 at Bath, though he played a lot there with New South Wales. With the Waratahs he played more at 12.
"Shaun is a very smart rugby player. He has been maligned over time that he can't do this, or he can't do that. But knowing Shaun and going back a long, long time with him - probably ten or more years - I know how skilful a player he is.
"I think he showed that at the weekend, not just with the fact that he's able to run the ball, but some of those glimpses where he knocked the ball down the right touchline, down the left touchline and back over the wingers' heads (was) smart play.
"And I think we needed that at that point in time. We needed to be able to mix our game up at that time.
"When Redser left Ireland I thought that he was technically the best passing nine in the country...and the service that he is giving at the moment was instrumental in us getting that performance."
Saturday's match is a sell out at the RDS and there will be no tickets on sale on the day.
In addition to the usual 'Family Day' activities there will be a number of themed seasonal events for Leinster's final home game before Christmas.
Alexandra College Choir will be getting supporters in the festive mood around the ground in aid of Fr. Peter McVerry's 'Welcome Home' charity.
LEINSTER (v Scarlets): Rob Kearney; Shane Horgan, Brian O'Driscoll, Gordon D'Arcy, Isa Nacewa; Shaun Berne, Eoin Reddan; Cian Healy, John Fogarty, CJ van der Linde, Leo Cullen (capt), Nathan Hines, Kevin McLaughlin, Sean O'Brien, Jamie Heaslip.
Replacements: Bernard Jackman, Mike Ross, Stan Wright, Malcolm O'Kelly, Stephen Keogh, Paul O'Donohoe, Fergus McFadden, Girvan Dempsey.