The actress, who played paramedic Comfort Jones on the BBC One medical drama, is expected to start filming her new role as brothel owner Michelle later this month.
Michelle has been tipped to cause chaos on the Chatsworth estate by recruiting its male residents to work as gigolos.
It is believed that Jamie Maguire (Aaron McCusker) is the first character to accept her offer, with his brothers and friends later following suit.
A source told The Sun: "Shameless has always been raunchy, but Aaron's scenes are steamy beyond belief. It's going to turn the show on its head."
Laird quit Casualty in 2006 after five years with the programme.