“Sirens” star Michael Mosley has joined Rob Reiner’s “LBJ,” portraying key political operative Kenny O’Donnell opposite Woody Harrelson’s Lyndon B. Johnson, Variety has learned exclusively.

O’Donnell was a close friend and adviser of President John F. Kennedy as part of his “Irish Mafia.” After Kennedy’s assassination, he served as Johnson’s aide from 1963 to 1965 and was campaign manager for Robert Kennedy’s 1968 presidential campaign.

The cast includes Jeffrey Donovan as John F. Kennedy, Jennifer Jason Leigh as Lady Bird Johnson, Michael Stahl-David as Bobby Kennedy, Richard Jenkins as Richard Russell, Bill Pullman as Ralph Yarborough and Kim Allen as Jackie Kennedy.

Reiner will direct from a script by Joey Hartstone, which follows Johnson’s ascension from the poor hill country of West Texas to the corridors of power in Washington. Filming is slated to begin in late September in New Orleans, Dallas and Washington, D.C.

“LBJ” is being produced by Acacia Entertainment, Savvy Media Holdings, Castle Rock Entertainment and Star Thrower Entertainment. Matthew George is producing the film alongside Reiner, Liz Glotzer, Michael R. Williams, Tim White and Trevor White.

Mosley has a recurring role as serial killer in ABC’s “Castle.” He also appeared on “30 Rock” and the short-lived “Pan Am.” He’s repped by Gersh.