Mrs Brown's Boys star Rory Cowan has said he is "over the moon with excitement" to be joining Fair City.
The Irish actor has been cast as John Bosco Walsh, whose first scenes will air on Sunday night's episode of the soap.
Bosco, Molly's brother, arrives to the O'Briens' house with Rose in tow because as Rose's dementia has progressed, Molly can no longer care for her.
The storyline regarding Rose's dementia is particularly poignant for the actor, whose own mother, Esther, also had the condition.
Speaking ahead of his Fair City debut, Cowan said he has been a fan of the show "since the very beginning".
He said: "But for a good few years, when I was touring the world with Mrs Brown's Boys, I lost touch with what was happening in Carrigstown. But when I left Mrs Brown's Boys to care for my mother, who had dementia, I got back into watching it again.
"Myself and my Ma used to watch the omnibus on Sunday afternoons. I've lovely memories of those Sundays with my Mam watching Fair City."
Cowan said of joining the soap: "The other cast members gave me a lovely welcome when I called in to see them. I feel right at home in Carrigstown. I'm delighted with the move there.
"Besides, if you're going to join a show, after you've been in a huge TV show already, then what's better than joining the biggest drama series on Irish television."
The 60-year-old Dubliner admitted that doing drama is "totally different" to his comedy acting on the sitcom Mrs Brown's Boys.
"In Mrs Brown's Boys Rory was a huge character with big camp reactions to any given situation. And on stage his reactions had to be even bigger," he explained.
"In television drama you've got to bring everything back and make your performance as real as you can for any number of situations your character finds himself in. The timing and pace is different. It's another skill altogether.
"So acting in Fair City is totally different to acting in Mrs Brown's Boys, but that's what I find so interesting about it.
"If I'd been asked to go into another situation comedy I probably would have said no, but being offered a role in Fair City was never going to be something I'd refuse."
Cowan's first scenes air on Fair City on Sunday, September 8.
Fair City airs Sunday, Tuesday, Wednesday and Thursday on RTÉ One at 8pm.