The quartet are named in a squad which also includes Dominic Ryan, who is listed amongst the replacements after a three-and-a-half month absence through injury.
Kevin McLaughlin captains the team for the second time and he is named in an all-Ireland back row alongside Shane Jennings and the returning Rhys Ruddock, who starts at number 8.
Damian Browne again partners Devin Toner in the second row, while Jack McGrath makes his 15th appearance in a front row which includes Cronin and Jamie Hagan.
In the back-line Eoin O'Malley partners McFadden at centre, with scrum half Reddan linking up at half-back with Ian Madigan, who passed the half century points mark for the province last week when he scored his seventh try in 15 games.
Heinke van der Merwe is in line to make his 50th appearance for the province should he come on as a replacement. The South African prop has been named on a bench which includes Isaac Boss and Richardt Strauss.
Ticket sales for Friday's game have passed the 16,200 mark. Tickets are on sale online here and from the Leinster Rugby Store (Donnybrook) and Spar (Donnybrook) as well as Ticketmaster outlets nationwide.
"This time of the year is very disjointed; both in terms of player availability and scheduling," admitted head coach Joe Schmidt.
"We are pleased to welcome back a few of our internationals this week though their training time with us has been limited. Having said that, the quality they bring will hopefully help us to keep some of the momentum from last week's Treviso second half.
"The Scarlets have proven that they can win on the road which makes them dangerous opponents, even though they will also be without some of their international players.
"They play a direct and attacking style of rugby with a lot of speed and movement in their game.
"Nigel Davies has the team playing in a really positive manner after a tough start to the season and they now sit just outside the semi-final spots - further motivation for them to come to the RDS looking to add a further four points to get closer to play-off contention."
LEINSTER (v Scarlets): Isa Nacewa; David Kearney, Eoin O'Malley, Fergus McFadden, Fionn Carr; Ian Madigan, Eoin Reddan; Jack McGrath, Sean Cronin, Jamie Hagan, Damian Browne, Devin Toner, Kevin McLaughlin (capt), Shane Jennings, Rhys Ruddock.
Replacements: Richardt Strauss, Heinke van der Merwe, Nathan White, Leo Auva'a, Dominic Ryan, Isaac Boss, Noel Reid, Brendan Macken.