The pay cable network made the announcement Saturday during its portion of the winter TCA press tour.
"Borgias" is set in Italy in 1492 and follows a powerful crime family headed by patriarch Rodrigo Borgia (Irons).
"I can guarantee you've never seen a family quite like this before, and nor could you make up the outrageous twists and turns of their epic saga," Showtime entertainment president Robert Greenblatt said.
Michael Hirst, who wrote every episode of "Tudors," will run the project, which originated at DreamWorks TV and Robert Zemeckis' ImageMovers. Neil Jordan, who has been trying to get a "Borgias" feature off the ground for years, will executive produce and direct first two episodes.
Production on the series is slated to begin in the spring for an early 2011 debut.
"Borgias" is one of several new Showtime scripted series, including recently ordered Laura Linney cancer comedy "The Big C" and the Matt LeBlanc-starring comedy "Episodes," about a U.S. remake of a hit British comedy to be toplined by the "Friends" star.
"Episodes" got a strong first reaction from critics: A trailer for the show, in which a graying LeBlanc must audition for his own part against a room of leather-jacketed Joey look-alikes practicing his trademark line "How you doin'?," had the TCA audience in stitches.
James Hibberd contributed to this report.