Monaghan, whose credits include starring roles in the thrillers "Eagle Eye" and "Source Code," will play Maggie Hart, the wife of Harrelson's Martin Hart character. Her character is described as "a woman who makes a hard decision that has long-reaching and devastating consequences."
The anthology series, which stars McConaughey and Harrelson as a pair of detectives whose paths cross while they're trying to solve a 17-year-old murder in Louisiana, will start with the 1995 murder and pick up with the case being reopened, with the first season wrapping when the case is solved. Subsequent seasons of the series will center on new characters and stories, much like FX's "American Horror Story."
Deadline first reported the news of Monaghan's "True Detective" casting.