In an epilogue in the fourth volume of The Boys, set years later, a more heavyset Cassidy (now going by his first name) has finally opened his "The Grassy Knoll" bar (albeit in New York City instead).

Not wanting to explain himself to his family or comrades, Cassidy decides to travel to the United States, living there in squalor for almost a century, figuring out how to exist on alcohol and drugs instead of blood, but inadvertently making him parasitical and irresponsible with a remarkable lack of forethought, causing harm and death to those around him when he abandons them to circumstances he set in motion, leading many former partners of his to end up critically injured, hopelessly addicted to drugs, or dead.

After he gets the girlfriend of a friend of his killed during the "Dixie Fried" story arc, Xavier, who is a Voodoo priest, says, "I honestly don't believe he's an evil man.

Over the course of their adventures, Cassidy forms a strong friendship with Jesse and eventually falls in love with Tulip, though she initially does not reciprocate the feelings.

Jesse is revived and goes looking for Tulip, while the redeemed Cassidy watches his first sunset in years as a human being, then drives off with a pledge to act like a man.

After ejecting a group of drunk Irish-Americans dressed as Irish stereotypes from his bar on St. Patrick's Day by throwing an axe at the wall behind them, Cassidy returns to his ongoing conversation with his British friend and former drinking buddy Billy Butcher about both being recovering alcoholics, and reminiscing about times in the past when they would have gone out drinking and acted like as "Stage Irishman", before both drink club sodas, toasting to the Alcoholics Anonymous mantra of taking it "one day at a time", serving as each other's sponsor; Cassidy is notably the only character in The Boys to address Butcher simply as Billy, while Butcher addresses Cassidy simply as Proinsias.

In the first season, working on a corporate jet as a flight attendant, Proinsias Cassidy drinks with a group of hedge funders while telling them about his last trip to Tijuana.

That night, after Jesse passes out from taking a drink from Cassidy's flask, two angels, DeBlanc and Fiore, arrive seeking to retrieve Genesis.

After learning from Jesse of his newfound abilities of control over others, which he believes to be the "Word of God", Cassidy is tracked down by DeBlanc and Fiore, who explain to him the true nature of Genesis and how they must return it to Heaven.

Acting as Jesse's representative without his knowledge, Cassidy agrees to sell Genesis back to the angels in exchange for copious amounts of money, drugs, and alcohol.

After failing to convince a distracted Jesse about the angels and Genesis, Cassidy elects to do so the following day, proceeding to a local brothel to engage prostitutes.

Sheriff Root realizes Cassidy is a vampire and attempts to use his immortality as torture to find Eugene's whereabouts, believing Jesse killed him.

The following Sunday, Jesse uses the angels' phone to summon an image of God to his church, which Cassidy, Tulip, and the congregation remain in awe of.

After escaping the Saint of Killers again, Cassidy recalls having seen Fiore on television, now working a novelty act where he kills himself and comes back to life at an Indian-style casino and hotel.

[11] After Jesse uses a fragment of his own soul to control and imprison the Saint of Killers, he, Tulip, and Cassidy participate in a game where people get shot at while wearing a bulletproof vests, and they trick the other players in order to rob their money.

Afterward, Denis tells his father that he is dying from congestive heart failure and wants to be turned into a vampire to survive, but Cassidy declines.

As Jesse forms a truce with Grail leader Herr Starr and realizes the man in the dog costume whom he had rejected was, in fact, the actual God in disguise, Cassidy bonds with his son, who appears to have no control over his vampire desires.

[15] In a flashforward opening to the fourth season, Cassidy and Tulip meet in a hotel room, where they talk about Jesse's death and kiss.

Months earlier in the Middle East, Jesse and Tulip invade Masada to rescue Cassidy, who is being regularly tortured by American mobster Frank Toscani, who teaches "advanced torture" at the "University of the Grail" by repeatedly cutting off the regenerative vampire's foreskin, using it as a key ingredient in face creams (a conspiracy theory Cassidy had championed, ironically proven true).

[17] On returning to his cell, the Archangel asks Cassidy why he doesn't struggle anymore, and Cassidy thinks back to when he was an Irish rebel during the Easter Rising: after his son was killed by British soldiers, he fled from Dublin, before being attacked and turned into a vampire on his way back home, leaving him resigned to not return to his family.

[18] Elsewhere, Cassidy is rolled to the helipad, preparing to bring him to a cosmetics factory in Bensonhurst for repeated industrial torture before using one of the angel's feather to pick his locks, break free, killing his guards, and Frank.

Being mistaken for a Grail agent, Tulip is assigned as a valet to visiting celestial emissary Jesus Christ, who, sensing that she is in trouble, agrees to help her free Cassidy from the dungeons.

Months later, God finally tells Herr Starr where Humperdoo is, and Grail agents snatch the Messiah from Tulip and Cassidy and return him to Masada.

[26] In the second audiobook, Michael John Casey voices Proinsias Cassidy, adapting his epilogue cameo appearance from the G-Men arc "We Gotta Go Now".

On being revived as a human, Cassidy becomes heavyset (with strong arms) over the years, growing his hair out into a ponytail, wearing a Father Ted t-shirt, and wielding a small axe to keep out rowdy Irish-Americans acting like "Stage Irishmen" on St. Patrick's Day.

"[31] Speaking of his portrayal of Cassidy, Gilgun remarked that "They let me by myself",[32] while furthering that sentiment by stating, "They let me run riot and be a total a–hole and do my job [...] There's not the right words in the right order to describe it.

You're the character that is written on the page'... You could tell Joe's lived like 100 lifetimes, and he's probably done some shit you do not want to hear about, but at the same time, he's one of the most fun, loving people you'll ever be around.

Gilgun spoke with The Wall Street Journal in an interview about the filming of Cassidy's 30,000 feet plane-fall scene from above a small Texas town "It's his introduction, so it has to be big, it has to be fast, it has to be exciting.

Everyone thought we were joking [...]",[36] clarifying that it was only after bringing up the matter, three days later, questioning how such a feat was going to be achieve that "[...] people realized that yes, we're dead fucking serious here", before describing his favorite scene in the show to be the one simply involving Jesse and Cassidy sitting in a jail cell and arguing over the nature of faith, along with the concepts good and evil, commenting, "One of my favorite things about the books were the bickering theological discussions the characters would have.

[38] On June 17, 2016, Monster Swamp artist Dustin Nguyen created a custom Preacher comic cover featuring Cassidy after having recently fed.