The Irish actor will play vampire Hal, replacing Aidan Turner's character Mitchell who was killed in the last series finale. However, he insisted that his character will be very different to Mitchell.
Molony told SFX: "I'm actually playing an English vampire - very, very posh. Very upper class.
"Hal's not Mitchell 2. There's a lot of backstory to play with, and I assure you he's very, very different."
Molony added that Hal will dress smartly, saying: "At the start, I'm a tweed suits man. Very, very well dressed - pressed shirts and cufflinks.
"The best part about playing a vampire is you get all the best clothes. I know some people have raided Aidan's wardrobe from last year and helped themselves to some coats and some jackets."
Being Human creator Toby Whithouse previously teased the fourth series, claiming that the show will take a journey "to somewhere even we have never gone before".
A former guest star has also confirmed that he will reprise his role on the supernatural drama.
Watch a behind-the-scenes feature for Being Human's third series below: