The first images from the series shows a remarkable likeness from a profile angle between the president and The Gangs of New York and In Bruges actor.
The Dublin-born 65-year-old joins a relatively short list of actors to portray Mr Trump, with Alec Baldwin's impression for Saturday Night Live being the most well known.
The series, timed to follow the results of the election between Mr Trump and presumptive Democratic candidate Joe Biden, is based on Mr Comey's New York Times bestselling memoir, A Higher Loyalty, plus an additional year of interviews with key players from the events leading up to the 2016 presidential election and its immediate aftermath.
"The Comey Rule is not a biopic of one man, but is instead the story of two powerful figures, Comey and Trump, whose strikingly different personalities, ethics and loyalties put them on a collision course," Showtime said in its announcement.
The first four-hour part of the series will focus on the Russia investigation, Hillary Clinton's emails, and the night of the 2016 election.
The second four-hour part will give a day-by-day account of the relationship between Mr Comey and Mr Trump in the first months of the presidency.
The cast will also include Holly Hunter as former Acting Attorney General Sally Yates, Michael Kelly as former FBI Director Andrew McCabe, Scoot McNairy as former Deputy Attorney General Rod Rosenstein, Jonathan Banks as former National Intelligence Director James Clapper, Oona Chaplin as former FBI lawyer Lisa Page, Amy Seimetz as former FBI lawyer Trisha Anderson, and Steven Pasquale as former FBI agent Peter Strzok
Peter Coyote will play Robert Mueller while Kingsley Ben-Adir will play President Barack Obama.