It's Joseph Gilgun, the British actor who you might best know as Rudy in Misfits. He's English, not Irish, but at least he has Cassidy's gaunt look. He joins Ruth Negga as Tulip and Ian Coletti as Arseface, leaving only the lead, Jesse Custer, remaining.
And if you were wondering, "Does Deadline also have a bizarre character description of Cassidy?" I am happy to report the answer is unequivocally yes.
Jesse's sidekick Cassidy is described as the most wild-ass, bestest "bro" you'd ever want to meet. He may be 100 years dead but no one's more boisterously alive than Proinsias Cassidy. An incorrigible mischief-maker, Cassidy's up for anything — joyriding, bungee-jumping, bank robbing, peyote dropping. He's also a relentless conversationalist with opinions on everything from religion to politics to pop culture to theories on why people are better able to tolerate the odor of their own flatulence.
The word "bro" used to describe Cassidy fills me with mortal terror, but I'm reasonably confident this is network-speak, and not the casting call that Seth Rogen and Evan Goldberg sent out.
Contact the author at firstname.lastname@example.org.